I have been reading alot of things, and was thinking of putting DS(3 in July) in a booster seat. Since his just over 14kgs and they're from 14-26kgs. But just reading the goverment transport sites saying booster seats are for children 4+yrs. Did you put them in when there 14+kgs, or 4+yrs? His carseat we have is now too small. (we only use it in family members cars as neither of us drive). We have a cushion booster seat. Is that ok to use for him?