Yes, you most likely do have to pay, and if you don't pay, most likely they could recover the money in court, if they chose to sue. Someone is liable, or financially obligated to pay, if they are negligent, or careless, and damage another's property. A parent is under a duty to keep on eye on his/her children and prevent them from doing things that lead to property damage; by not preventing your son from running into a room in a strange house and locking the door, you would most likely be considered to have been negligent and therefore at fault, so liable to pay.