I've just got Liv a Safe and Sound Maxi Rider AHR. It has a fully convertable harness which can take a baby from 8kg to 26kg. The AHR is "Active Head Restraint" which is for side impact head protection incase of an accident and it is adjustable in height so they can be in the seat for longer. It exceeds safety recommendations. On a side note it's so cute with it's little arm rests and cup holders. Liv loves it and no-one is allowed to put her arm rests up or down except her. She is so protective. Have a shop around darl and research what works for your DD.
There are the foam booster seats available but they have to be over 14kg to use these.