Why not invite them around to your house to discuss it but secretly record the conversation? Over here you do not have inform anyone that you are recording them so long as it is your own home. This may well be enough to provide police with sufficient grounds to begin investigations, perhaps even lead to prosecutions. I would not see this as revenge but as justice. Of course, this may not be the best thing for your child. I f****ng hate those b*****ds.
I?m very sorry for your child and your family.
Hugs