If the problem was bad design or manufacture--that is, the mattress was dangerous as designed or made--you may have a product liability claim against the manufacturer (and possibly also the seller); if so, you could potentially recover your out-of-pocket (not reimbursed by insurance) medical costs; pain and suffering for your daughter, especially if there  will be any lasting pain or impairment; and lost wages (if you missed work to care for her). On the other hand, if the mattress was not defective but had been damaged in some way after you purchased it, and that's what exposed the wire, there would be no liability.