Your poor DD. Don't beat yourself up over it, it has happened and she will heal quickly.

In light of 'other accidents' happening while he is around, it doesn't sound like it was an accident to me and I wouldn't be having the kids around eachother either. I also tend to think that the other kids aren't telling you how it happened because they're scared of what he might do to them if they did. From my own knowledge of kids, if it was a pure accident they would've told you exactly how it happened when you raced into the room, they'd probably even act out the scene lol.

Sounds to me like the boy needs some form of counselling.

I got a spiral fracture on my leg when I was 4 by falling down a firemans pole in a playground. I fell about 5 metres or so, so I would hate to think of the force it would have taken for your DD's leg to break like that.