One thing to keep in mind is that children, especially younger children, still think that life revolves around them, and that is appropriate for their stage of emotional development. The tricky part is that then they may, without expressing it, think that it is their fault that mama only calls sometimes, and if only they were better, kinder, prettier, happier, put your word of choice here, mama would be here.
In the circumstances you've outlined, I see every reason for you to cut out random contact when it occurs to their mother to remember that she has children. If and when she wants to be a responsible parent, the Court has set up a way for her to proceed.
You don't have to try in any way to "fill in for her" or keep a non-existent relationship between her and her children going, for pretend sake. Shakespeare said "true compassion is ruthless", and the truth is that they don't have anything beyond a biological mother.
The truth is that children are worthy, and are entitled to feel self confident and happy, and that may mean telling them the truth that their mother has a disease that keeps her from being a real mama. It is a hard lesson, but a freeing one for them to be able to accept that they can be worthy even if their mother isn't, and that they are not to blame for her behavior, and they cannot change it.
ShootingStar1