Parental "informed consent" for underage glider flights

I was talking to a big fan of motocross about a 13 year old who became a quadriplegic after a motocross accident.

What is the difference between an adult consenting to taking the risks of flying and a parent assuming those risks for a child?

My dad let me do plenty of dangerous stuff when I was a kid, but he would occasionally draw the line and say no.

How do people sort this out?

When an adult asks me whether soaring is dangerous, I will tell them that I know of two cases in the last ten years where a person died on their first glider flight. No one has yet to cancel their plans when I told them that.