I have no idea about support issues, and I am sure others will come along soon with their experiences. I just wanted to say I am sorry, but good thing you got the kids and can give them the stability and consistency they need.
I do know our state deals with divorce issues and custody issues separately, but I am surprised she was not ordered to pay any child support. Does she have a lot of visitation? Was there a reason she wasn't given even a small amount to pay? My friend was ordered to pay $478 per month for one kid, even though at the time she was unemployed.