Oh, Ad,
IDK, I wouldn't be trying to make this about paying money for the other kid or his parents.

Other kid is probably horrified by what happened as well. He hurt his friend with a silly unintended act.

It was an accident, he is a kid... He has already learnt a harsh lesson.

Again, just my 2c. (& in the context of knowing nothing about your hospital or health insurance system in US).