not sure if anyone else has mentioned it, but you can get a booster seat that is actually suitable from 8kg's all the way until they're 7 years old, it has the 5 point harness in it still, (which you take out when they're too heavy/too old) so it is essentially still a carseat, but you dont have to worry about the safety in that sense! and your DD can use it when your DS grows out of it

there's plenty of different ones, but i know there's one in the Maxi-Rider range, but i just looked on baby kingdom website and they have a few, ill PM the link to the different ones, as im not sure if im allowed to post it?

HTH