I am a law school graduate currently awaiting Bar results. What I offer to you is mere information, not to be construed as forming an attorney-client relationship.
Well, i am not sure what mississippi means. I thought child support ended when the child became of adult age (18). however, mississippi may have the age be 21? Emancipation usually occurs either when a minor legally emancipates or "divorces" his/her parents OR when the child is an adult OR is married.
If you cannot afford an attorney, your best bet is to contact your local legal aid foundation and ask for the family law section. Since they deal with issues of present and back child support all the time, they would probably be able to tell when the legal time for stopping payment is.
However, I should say one thing: your husband may not have a legal obligation to support or help his children in a monetary capacity, but perhaps he should feel like he wants to help his children.
Lots of luck to you.