Even a nerf ball can hurt if thrown hard enough. Especially to the head.
This is so freaking retarded. They want kids to get more exercise, yet you limit what they can do in fear that they'll get hurt. Might as well ban gym altogether because, hey, a kid could get hurt running. Or all sports like baseball, basketball because a kid could get hurt. They're setting an extremely bad example for these kids. People already hate exercise; it's not fun. Take away activities where kids do get exercise then they'll never want to exercise.
If they couldn't get the kids to stop throwing balls in the wrong direction, like towards the daycare, then you should be punishing the child(ren) who disobey like not being allowed to play during recess.