Hi Aimee, gosh your Hannah is good, my guy is just a featherweight LOL. You're right to want to keep her harnessed as long as possible, as she's still very young for an adult belt. You have a few options, depending on what you want to pay and how it fits in your car. I know Bonnie will post in this so I'll keep mine brief lol.
Options 1, 2 & 3 from the Extended Harnessing page:
The inbuilt harness goes to 18kg then you take that out and thread a special 'Protecta' harness through the top slots and clip it to the tether, so it acts as close to an inbuilt harness as you can get. That will take her to 26kg, and costs around $300ish.
If you're absolutely stuck then you could combine a child harness with a Pegasus booster that has a submarine clip to hold the harness down at the crotch, but it won't be as snug a fit as the others above. Hope that helps?
