I have two boys, 9 and 7 and two girls, 2 and 6 mo. My wife is a stay-at-home mom and I work usually 50-60 hours a week. The two boys are pretty much free range kids. They do what they want. I kick them out of the house, tell them to go ride their bikes. Sad though, there's rarely ever other kids outside for them to play with.
Youngest son currently has his arm in a cast. Fell off the playground equipment joking around with his brother.
Eh... It happens. Broken arms. He'll learn to be more careful. That's how we all learn. Or at least that's how kids used to learn before the world was covered in bubble wrap.
And maybe those boys are allowed to range freely because of your little-girl spares!