wow, I am not sure how these things truly are handled. The important thing to me is: Make sure the children know that you care for them, maybe even love them as we are not sure of your relationship with them.
The sad thing is this: the mother may never ever make her choice to go back to these children. No matter what, and even hitting 'rock bottom' does not always make it clear to us.
Make sure you alert whomever it is that is in charge of the case, as this may be important.
But I feel these children have gone through enough (perhaps more than you are even telling us, as there is a 'man' in their home with their mother and they were removed means so much more is happening to their delicate souls!). Continue on being involved with these children, help them and know that it is not only children we touch: everyone comes into our lives for a reason.
I hope this helps!