You may be liable. Parents are liable for intentional torts, or deliberate injuries or damages, caused by their children. Pushing someone is assault, an intentional tort; therefore, you could be liable for the medical costs, etc. arising from the broken tooth, should his family elect to sue. While any parent (or anyone who remembers being a child) would sympathize with your daughter, the law does not make being made fun of a defense to assault (if the bully had pushed or hit her, that would be a different story--then it might be self-defense).