For those who arent aware:
Children upto 3yrs must use appropriate child restraint. (baby seat etc)
Children between 3 - 12 must be in a booster seat and use the correct child restraint.
I think thats about right... it goes on childs age and height as to when they can just use a normal seat and adult seat belt (12yrs + or 4ft 5ins)
Theres more to it than that though, regarding taxi's etc.