E.S.
I think it depends on the state you live in. In TN, the children from the first marriage, if given child support, can not have their support lessened because of children that come later with someone else.
My lawyer used the following example:
A doctor left his wife of 15 years and paid $1000 per month for child support. He remarried, had a child and divorced. His income did not change during all this time. The child support for the child from the 2nd marriage was figured using his income MINUS the support he pays to the first wife. The second wife, got conciderably less in child support in relation to what the first kids get.
The girlfriends income does not count.
He can always petition the court for a reduction in support payment, but he would have to show that he is making less money, not that he has more kids.
BTW: Most lawyers will answer a few questions for free and you can always contact your lawyer from your divorce. Also, legal aid can help.