It's still quite rare that this kind of things happen when you let your children go outside with their friends.
I have no experience from parenting, but I still think it's good to allow your children to go play outside with someone else than just parents.
...They can't stick with their parents all the time.
And they're school age already.
But well... It's really rare that this kind of things happen when you let your children go outside. Most of the time when kids get any kind of injuries, they're just minor.
...
I wouldn't go blaming the parents immediately.
Even though I agree that nowadays way too many parents think no disciplining is the way to go. It's annoying to see parents in a mall that don't stop their kiddos crash all the plates.
Having order makes makes them learn something...