Continued...
The good thing about our position is that because the children were taken away by the state, there is no formal agreement that the parents will have anything to do with their children. That is completely up to us. Right now, we do not communicate with the parents. Maybe when the children get older and express interest in seeing their parents, we will think about it, but not right now. My brother and his wife were in a similar situation, where they had partial guardianship of a young girl, except the state did not take the child away. There was an agreement between my brother, his wife and the mother through the court system that the mother had the right to spend a certain amount of time with her daughter. Well this turned out to be an extremely bad situation that ultimately ended in my brother and his wife having to necessarily cut their ties with both the mother and the little girl and threw my brother into a deep depression that I think had something to do with his early death (yes, the situation was that bad).
So, if you can adopt the child with the parents consent, that would be the best option. If you do adopt, you can still let the parents have contact with their child or not at your discretion. Seriously, beware of any official agreements giving the parents a right to see their child. People that lead the type of lifestyle that you describe above are unpredictable and the situation can turn really ugly very fast.
Let me know if you have any specific questions. The laws about adoption are different for each state.
Good luck!!