Because there was a divorce and that judgment apparently found husband liable for the children's support, there is no way to attempt to hold the children's mother or the natural father liable, because the dissolution judgment is "res judicata," i.e., a final and complete expression of the law of the case.
A judgment may sometimes be set aside on grounds of mistake or fraud, but it would be very difficult to accomplish on your stated facts, because family courts lean HEAVILY in favor of minor children and they will bend over backwards to avoid creating opportunities for disgruntled parents to rehash a child support action.
Also, different State jurisdictions have very different rules about challenging paternity. In some jurisdictions, after a certain number of years, no challenge is possible under any conditions. So, a lot depends on where the divorce was obtained.
If you really want to try to set aside the divorce judgment as applies to the children, you will need a very competent family law attorney in your locale. It could be quite expensive.
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