Daycare Liable for injury sustained at their facility?

Additional information
This happened when my son was 17months old. He was "promoted" from a baby room to a toddler room. He went from sitting in a high chair to a "regular" chair but smaller. They had chairs in the room that could buckle him in but that is not the chair he was in. They left him unattended and he started to fall asleep. At that point he fell out of the chair and fell into another chair that was next to him. He smacked his mouth on the metal arm of the chair and his front tooth was broken. We came to an agreement w/the center director that they would take care of his dental bills until we were sure that the adult tooth was not affected (which would be until he is at leat 7yrs old). The tooth turned gray and died. It abcessed and we needed to have it removed this past Feb (not long after my son turned 4). This was the point when we learned that they would no longer pay for any bills. We were told that the insurance no longer covered the injury.