I think they should get another one(as cheap as can be found), put a tracking device in it, leave it out in plain sight and then when the newest leg is stolen, the police can track that leg to the low-life scum who took it. Keep the good replacement leg elsewhere in the house, in a safe or under lock and key with another tracking device inside(just in case they find the good leg).
If I were in this family, I think I might be scared that someone was targeting my family and someone sure knows a lot of details about the family's life including where this girl's bedroom is...I hate to think it's someone they know.
agnes!