Ontario Booster Seat Regulations Just Wondering How Tall A Child Should Be To Use A Booster Seat?

Just wondering how tall a child should be to use a booster seat? - ontario booster seat regulations

I know your 40 pounds or 4 years, but my son is only three, but is very high, weighs only 34 pounds ... are not as big as the car seat b / c hes a result of a recent car seat, in the wonder if its security to change or not? I live in Ontario, Canada if that makes any difference?