I have it right in my contract :
We agree to paying 100% of replacement cost for any damages that may occur do to intentionally damage of my property through destructive behaviour, roughness or due to a breach of one or more of my policies listed in this agreement or on the daycare website; XXXXXXXXXXX.weebly.c om. And the reimbursement will be paid within a reasonable time frame.
I would also do time outs and lose of toys/movie etc.

If it was just an accident then no cost to the parents or punishment. Toys do break.

You told the kids to calm down they where being to rough, I would have the parents pay to replace it. I would also have the kids apologize to your son for breaking the toy.