Not having a job is not an excuse for withholding child support. The checks you are getting are probably the SSDI payments due to your children because their father is on SSDI. If that amount is not covering the court ordered child support payment and if he hasn't paid his 1/2 of the medical and incidentals, then you can go back to court and file a Complaint for Contempt against him.
The court will take the amount he owes and add it to any arrears that he must pay back. Let the court know that he does work under the table.
Your ex should have filed for a modification. Having failed to act, and incurring an obligation, I agree with Gabe that a complaint for contempt should be filed. The Court can order him to look for a job.