I have been a grandad taxi for a few years. The littlest one is borderline nearly to a booster move. Vlad makes the correct point though legislation keeping it all legal it is down to the size of the child their are height requirements and if they come within that you don't need anything. Boosters as you know are to let them see a bit more and to be more comfortable. You will find the middle seat is usually convex and they like the boosters to be on the outer seats they are bit less convex and are behind a seat presumably less chance of getting thrown through windscreen.