Children who outgrow child restraints with harnesses either in weight or height should use a belt positioning booster seat with a three point lap and shoulder belt. Booster seats serve as an important step between a restraint with harness and an adult lap/sash belt on its own (without a booster seat).
Research has proven that using a belt positioning booster seat instead of a seatbelt alone reduces the risk of severe injuryby 60%. Using a booster seat increases the protection from injuries in a crash.
Skipping the booster seat step or getting rid of the booster too early is a common but dangerous move.
Most children are not mature enough for an adult belt alone until they are 4' 9" tall (approx 148cm) which is usually between the ages of 8 and 12.
Children prematurely “graduated” to adult belts are more susceptible to injuries such as broken spinal cords, head injuries, brain injuries and ruptured livers and spleens. Boosters protect your child's bones and spine and keep internal organs like intestines, liver and spleen from being crushed. Children can also slip below an incorrectly fitted belt. Boosters also help reduce slouching, one cause of poor lap belt fit. Booster seats also reduce the misuse of the sash belt and reduce the risk of ejection.
“When children are “ prematurely graduated” to safety belts…, the lap portion of the belt rides up over the abdomen and the shoulder portion crosses the neck or face. This places the child at risk for “submarining” or sliding out of the lap belt during a crash and for injuries to the neck as a result of the poor fit of the shoulder belt” – F. Winston JAMA - Journal of the American Medical Association 281, 1999
On young children the shoulder portion of the adult belt often crosses the neck rather then the shoulders which is both uncomfortable and dangerous. To make themselves more comfortable, kids will often put the shoulder belt behind their back or under their arms – making it dangerous.
This simulation shows a child improperly seated in an adult belt with the shoulder belt behind the back.This child is thrown forward dramatically in a 56 kmph crash.The inappropriate fit of the seat belt and lack of upper body restraints puts the child at risk for severe head, spine, abdominal and brain injury.
This simulation shows how a 6 year old child properly restrained in a belt-positioning booster seat barely moves during a 56 kmph crash.
It is obvious to see that the child is much better protected because they are using an age, weight and height appropriate seat.
If there is no shoulder belt when a crash occurs, your child’s upper body and head can fly forward hitting the interior of the car causing neck, spine or brain injury. Without a booster, the lap belt sits high across the child’s stomach instead of flat on the thighs and the belt itself can cause internal organ injuries in a crash.
According to a news article by One News, researchers at Starship Hospital found that:
Every child aged 5 and under should be restrained in an appropriate child restraint
90% of children aged 6-8 (the age group with the highest number of injuries at Starship) should still be using booster seats
50% of 9-10 year olds should still be using booster seats and
10% of 11-12 year olds still require a booster seat
If you answered "no" to any of these questions, the child needs a booster seat to make both the shoulder belt and the lap belt fit right and to keep them safest.
What does the law say about how children under 148cm and 12 years of age should be restrained?
Currently (as of June 2008) the law in New Zealand is that :
Any child under five years is properly restrained by an approved child restraints. They must not travel in the car if you can't put them in an approved child restraints.
Children aged five to seven years must use a child restraint if there's one available. Otherwise they must use a safety belt if one is available. If there are no child restraints or safety belts available [see exceptions to the law], they must travel in the back seat.
This means that technically [unless you have a booster in your car for your 7 and under year old and refrain from using it] even if your child is under 148cm and doesn't fit an adult belt at all, you're not breaking the law. Unfortunately however, the law doesn't always mandate what's best, or what is safest.
As above, most children are not mature enough for an adult belt until ages 8 - 12 or 4' 9" tall (approx 148cm) height being the most important factor of course. Whether or not a child can be properly seated behind an adult belt without the aid of a booster seat is not dependant on their age and has little to nothing to do with what the law mandates. The law is the minimum of what is required of you to do. It is the beginning, not the end.
Carseat to booster - When?
"Every step in car seat "advancement" is actually a step down in your child's protection" - CPSafety
When your child completely outgrows their forward-facing harnessed car seat you can switch to a belt positioning booster seat. It is recommended that children remain in an approved harness of some form until they are at least four years of age. The forces are spread more evenly across the body in an accident. But with a three-point belt there is higher risk of ejection from the seat in an accident which can result in fatal consequences.
Most booster seats in New Zealand have a weight range starting at 14-15 kilograms with a maximum weight limit of 26-36 kilograms.
How will I know whether or not my child should be in a booster?
As above, a booster seat is for use once a child has outgrown their forward-facing (inbuilt) harnessed restraint. You will need to refer to your instruction manual as not all restraints have the same height or weight limits. The child in the picture below has just about outgrown her seat.
They have outgrown their child seat if the their eyes are above the top back shell of the seat or if they have reached the maximum weight limit for their seat. If this is the case, then some suggestions might be:
Find another car seat e.g. convertible booster with a taller back
Buy a booster seat and use in conjunction with a child harness if you would like more upper torso support. You may also want to look for seats with anti-submarine devices that prevent the child from slipping under the belt
Buy a good belt positioning booster seat (in which case it is particularly important you make sure your child is over 4, within the weight limits of the booster seat and emotionally mature enough to remain seated correctly for every journey).
Please remember: Booster seats have minimum weight limits, usually 14 or 15kg, make sure you read your instruction manual or stickers on the seat to find this information. If your child is below the minimum weight requirement of the booster seat, then it is not an appropriate nor approved restraint to use. However, there is more to using an appropriate seat then meeting the minimum weight requirements. Appropriate also means height wise, age wise and maturity wise.
Many children move in to boosters because their younger siblings need their seat or because their peers are in boosters too. Please don’t demote your child to a booster seat because it is needed for a younger sibling. Think about buying or renting another seat until they are physically ready for a booster seat. Don’t use what other restraints your child's peers are using (if any) as an indication that they should move in to a booster. Chances are other children might have prematurely ‘graduated’ to boosters themselves. Children are very much individuals in their physical maturity, weight, height and so forth. While some children may be ready for boosters, that doesn’t mean yours is.
Booster cushions are small and light making them easy to carry and because they tend to be relatively cheap (compared to high back boosters) they are ideal for keeping as a spare. This would be helpful for instances where it may not be practical or possible to transfer the childs main seat to another vehicle. I.E. family emergency, school trip etc. Most backless booster seats come with a shoulder belt positioner to adjust the shoulder belt height on the child.
Backless boosters must be used in vehicles with high seat backs or head rests. If there are no head restraints behind the head, the child is at risk of hyper extension. Hyper-Extension is a form of whiplash, when the head is snapped backwards in an accident i.e. if you are hit from behind. This can cause injury to muscles, ligaments, discs, nerves and the cervical spine. These injuries can have long term consequences such as life long disability, or death.
Unlike a high back booster, a booster cushion provides no side impact protection so try to keep your child in a high back booster for as long as physically (and safely) possible. Please note, an actual cushion is not appropriate, nor safe, to use as a booster seat. Only use a safety approved and tested booster seat for your vehicle.
Integrated Booster Seat
Integrated booster seats are booster seats that are built in to the vehicle by the vehicle manufacturer. Many Volvo's in New Zealand have such boosters. You must check your vehicle manual for the weight and height limitations for using this as a booster.
How does a booster work?
Protects children too large for a child restraint with harness but too small for a simple seat belt.
A Belt Positioning Booster (BPB for short) raises your child so that the lap/shoulder belt is properly positioned across the chest and thighs, ensuring that in a crash the belt will protect and restrain as it was designed to do.
Booster seats are not tightly installed in the vehicle as other child restraints are, booster seats are held in place by the child's weight and the vehicles lap and shoulder belts.
It is important to remember that you must never use a booster seat with a lap belt, but always with a lap AND shoulder belt. It is illegal to use a booster seat with a lap only belt. If you use a lap belt, the child will be folded in half and hit their head on their knees or the seat in front of them in a crash. Children can also suffer serious internal injuries if this happens. If only a lap belt is present in the vehicle, then you will need to use it in conjunction with a child harness.
High back booster seats must be used when vehicle seat backs are low or do not have head restraints. This type of seat (pictured to the left) provides head, neck and back support for the child. A lot of full boosters on the market also offer some side impact protection for the child. It's obvious to tell whether or not a seat offers some SIP (Side Impact Protection) as it will have some "side wings" surrounding the head.
Weight requirements for booster seats vary depending on what Safety Standard they meet. Check your instruction manual, stickers on your restraint or contact the manufacturer if you are unsure or can not obtain this information.
Even if the child is not present, secure the booster seat at all times so that it isn't a projectile. Children should always ride in the back seat up until at least age 13.
What to make sure of when using booster seats:
The shoulder belt goes across the centre of the shoulder and chest, not on the neck.
The lap belt should be high on the thighs or low on the hips, not on the tummy.
That you use a high back booster if your vehicle has low seat backs with no head rests. Some models may offer increased side impact protection.
From booster to adult belt
Please see the Five Step Test to see whether or not your child is ready for an adult belt.
Children who outgrow child restraints with harnesses either in weight or height should use a belt positioning booster seat with a three point lap and shoulder belt. Booster seats serve as an important step between a restraint with harness and an adult lap/sash belt on its own (without a booster seat). 
High back booster seats must be used when vehicle seat backs are low or do not have head restraints. This type of seat (pictured to the left) provides head, neck and back support for the child.