We are juggling this issue at the moment. Olivia is 2 but is not even 10kg yet, so we asked the advice of the Red Cross Safety Shop (who stock and fit car seats, capsules, boosters etc) and they recommended a "Safe and Sound Maxi Rider" by Britax car seat which turns into a booster, so you get 3 for the price of one?

Not sure if this is any help?