This is a surprisingly difficult area to get clear answers. The simple, but unhelpful, answer is ‘sometimes’.
If you are armed with the child’s age, height and weight, then you can get greater clarity. I have drawn up a table to try and help. Access it here. Read all of the table, to make sure you don’t miss an entry that applies to your child.
Whatever the minimum legal requirements may be, just get a child car seat appropriate to your child’s height or weight.