So, at a party at an animal park that your 7 year old son attended, your son had something of his own that another child wanted, when asked for it your son said "No" and the parent of the other child grabbed it off him hurting his wrist. You didn't realise at the time that it had caused damage, but the next day your DS admitted he was in a lot of pain so you went to the doctors/clinic/A&E. They did x-rays and scans, there was no fracture but there was soft tissue damage.
You've reported to the police and they are proceeding with an investigation but it's taking time (due to the animal park being in another force area).
You have spoken to the mother who assaulted your son and the host of the party. They are all minimizing what happened and (presumably because of the police investigation that threatens to criminalize the other mother) are turning on you and encouraging their children to bully your DS?
If I've got that summary correct, YANBU at all and I would be fuming and wondering what I could do too. But realistically there isn't a lot that will help. I would get a copy of the police report if you can and take it to the school to explain what 's been going on and that the mothers seem to be encouraging their children to bully DS. I would ask them to keep an eye out. I would encourage DS to make other friends and maybe enroll him in some clubs that are not mainly populated by his school peers. If it comes up in conversation with other parents I would outline what happened very basically and matter of factly and say that you can't say more because the police are involved (but practice this as, from this thread, it seems like you might have a hard time with it). If you are in a small school, I would think about whether a move might be a good idea - this needs careful consideration, but it may be the only way to get your son out of a difficult social situation that he cannot influence.
You need to let the police do their job - there is no other way to hold the other mum accountable, but I would put a bit of pressure on them. Other force or not, if they haven't even got around to interviewing 10ish weeks on, that's really too slow.
I hope your DS is recovered now and that things work out.