Not your state or a CS case but do not expect much by way of punishment for altering dates on a doctor's note. It doesn't get her out of paying support, just at best defers when those payments take place. If she hasn't been paying all along, then I don't see where this really changes anything for you. There is a reason it is unwise to have children with a drug addict and dishonest person. If this woman isn't in the children's lives, and they are being totally supported, then there should be minimal effect on the kids. The children have no reason at all to know the worst about Mom or what she does or does not pay for. If you are informing them of this, shame on you. That is what is going to harm the kids most. Sure it would be nice if she paid but with two "parents" already supporting them, they already have what most kids do or should. Most parents don't get child support and fully financially support their children. Nor do they send them off to someone else every other weekend. Treat it like what it is; two people raising a couple kids.
Hanging you hat on punishing this other woman or hoping that the mother of your children ends up in jail or even worse circumstances than she already is in, only serves to hurt the children and sets you up for disappointment.