If the arrears reach a certain level, the Domestic Relations Office will intercept his refund. Sometimes you have to call their office to make sure that they file the necessary paperwork. If you have primary physical custody of the children and he claimed the children as dependents without your having signed IRS Form 8332 giving him permission to claim them, you will have to contact the IRS to report this situation to them.
My response is based solely on the limited information contained in the question. It is not meant to substitute your attorney's advice.