Liviam at the age of 4 they can move from a 5 point harness to a booster. I have the Safe N Sound Hi Liner booster and am quite impressed, it is slimline and the head adjust ups as your child grows, it has a seat belt guide and also the anti submarine clip. I think though they recommend you don't move your child til they meet the weight thing. So around 18kgs is when kids should move from the 5 point harness as they are actually to heavy for the weight limit of the seat.

I hear you on being so tight they can't move one thing to keep in mind when using a booster is that you can buy a seat belt clip extender means you don't have to try and get your hand down between 2 seats to get the belt in. Has been great.