Talk to a lawyer if you want to sue the daycare.

Even you signed the waiver, if there is obvious negligence of the service provider, you can still sue them.

For your case, not really sure. Generally, a sharp corner should not be in a daycare classroom. Door handle broken by a toddler is not acceptable, either. You can check state regulations and etc. You should certainly make sure the daycare will remove/cover-up of the sharp-corner; fix the door handle. Have you talked to daycare to see if they are willing to pay for the medical cost?

BTW, if you do not like the daycare, or you decided to sue the daycare, you should transfer your son to another one.