Instead of basing on income, base it on wealth. Make him pay it down. It’s ill-gotten, so if anyone is to benefit from his and his dad’s dealing and stealing, let it be her and kids.
A couple things, based on California law (your results may vary). Child support must be based on income, not wealth, although the return on wealth is income for purposes of child support. Second, at least here, the presumption is that all court proceedings are open with limited exceptions like adoptions and juvenile. It’s tough to get stuff sealed. I can’t imagine a court here sealing the amount of child support.