Take the man to court.
It is unclear from your question whether or not there are orders in place regarding the children in your lives. Some people go to child support enforcement prior to taking any court action so forgive me for speculating a bit. If you can pull funds together to retain an attorney, you may be able to either obtain an order for support, or ask to modify. You can also pursue a contempt motiont aht will bring him to the court.
A family law attorney could really help here.
I wish you luck!
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Mr. Deasy is correct. You need to take him to court, whether through your own attorney or the County Attorney who support CDHS and subpoena the people who are paying your ex under the table to testify at the hearing.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.