DS is a rough one, I regularly grassed him up about some rough thing he'd done to other children (the kids complained to me when I worked as MSA).
1. She was playing along fine, if it was my DD I'd be telling her not to play tag with the boys if she can't suck it up if they get a bit rough.
Doesn't deserve to be pushed over, but I don't think that was main issue, it would have been a mild verbal rebuke to all to play more carefully if that was all. You never would have known anything about it.
2. DS swears blind he didn't kick her
He was part of a culture that encouraged it (well, probably). If you only hold kids to account for exactly what was precisely seen, nobody would be held to account for anything.
3. Nobody else saw the kicking, theres no evidence other than this girls report to her mother.
If she was your DD would you want her to be believed?
4. Where were the playground staff?
Understaffed as usual? Let their Back-of-Head-Eyes at home that morning?
5. I can't see that keeping 4 boisterous 5 yr old boys in all day is going to help anyone.
He'll remember to be more careful in future, to grass the others up if they do such a thing, not to be part of a culture that encourages it.
I've made DS write the girl a card saying sorry for pulling her over.
For that I honestly commend you.
Don't make a meal out of this, it's not worth it. Let him take it on the chin & move on.