Hmm I'm trying to understand here - are the fathers currently under a supervised visitation arrangement? Because if they aren't, you can push for that and, presumably, still retain the support agreement. After all, it's possible (even if it seems unlikely right now), that the fathers (or one of them) will straighten up and fly right in the future - supervised visits means that they would be able to maintain a relationship with the kids but you wouldn't have to worry about them being alone with the children.
Of course your concern is for the children over their fathers but I think that maintaining a tie - even a small one - could be beneficial to everyone in the long run. Also, there's counseling, and the presence of other role models in their young lives. For all of the bad or short or uncaring visits by the Dads, can you counter with better role models. What about Big Brothers?
http://www.bbbsa.org/site/pp.asp?c=iuJ3JgO2F&b=14576 They can help to provide positive male role models.