Sadly times are changing to the extent that children are being "wrapped up in cotton wool".
I had so much freedom in colonial Zambia and Kenya. If us children hurt ourselves when we played, we learned from our mistakes. Yes, sometimes we didn't need our parents' warnings and learned the hard way! But it was good for our development, learning through trial and error. It made us who we are today. We were taught not to be afraid of creepy crawlies, snakes, etc, but to have a healthy respect for all creatures ~ but best not touch in case it was poisonous or dangerous.
Us kids had a fantastic childhood because we were allowed to play, get muddy, have genuine fun.
I can understand your concerns regarding your grandchildren but, as other folk on here have said, let them be allowed to enjoy their childhood. Bumps, scrapes and bruises are part of learning what they are able or not able to do in various conditions. Support your son and DIL in their decision in allowing their children to play in the way they want to. And in need, provided a kiss, cuddle and a the occasional plaster.