The purpose of cars is to get people from place to place. That purpose isn't hampered by wearing a seatbelt. There's not even a compromise involved. It's all upside.

The purpose of childhood is produce future adults. That purpose isn't fully served when kids get too far removed from choices, risks, and consequences. Yes, we also need to make sure they stay alive, but survival can't be the only metric parents optimize for or they'll just produce confused old kids trying to live adult lives. (`Big`, but for real and without a magical fix in act 3.)