You do not report the delinquent child support to the IRS. You contact the local authority who oversees court-ordered child support orders to have them take care of it. They can inform the IRS and then the IRS can seize the ex's tax refund for delinquent child support. If your ex is not filing tax returns, there is no refund for them to seize. There is no tax return or IRS form for you to use for this situation.