You are not liable for damage to the sitter's car because the sitter's swing set fell.You can file a personal injury claim on behalf of your son against the homeowner's insurance of your sitter.When your son completes his medical treatment and is released by the doctor, obtain your son's medical bills and medical reports.  His personal injury claim filed with the sitter's homeowner's insurance carrier should include those items.Compensation for the medical bills is straight reimbursement. The medical reports will be used to determine compensation for pain and suffering which is an amount in addition to the medical bills.  If the case is settled with the homeowner's insurance carrier, NO lawsuit is filed.If you are dissatisfied with settlement offers from the homeowner's insurance carrier, reject the settlement offers and file a lawsuit on behalf of your son against the sitter based on premises liability.  You will need to be appointed guardian ad litem to file a lawsuit on behalf of your son because he is a minor.If the case is NOT settled, the lawsuit on behalf of your son against the sitter for premises liability must be filed prior to the expiration of the applicable statute of limitations or your son will lose his rights forever in the matter.