Tragedy of the commons happens when property rights are not clear. So one must make them clear to the children. You don't force them to share, you force them to conform to not hurting others or messing up their property, if needed. All else is education and freedom. You as a parent probably own the house. So you are kind of like a monarch, except you don't tax the child. You govern the rules of the house, but you let the kid learn how to truly respect himself and his fellows.