I ferry 2 kids to music lessons in my car every week. My child has a booster seat (backless). The other child's mum insists that its ok for the child to wear adult's seat belt. We did that for 2 weeks and now my child also wants to use just the adult seat belt. For some reason, I feel uncomfortable around this arrangement. Aren't they supposed to be using booster seats until 12? Or is it ok for 8 ?