People who knew her must have had some inkling.
The father did report to and SS and SS were involved.
It seems to me that he hasn't bothered with them for their whole lives, and now wants to exploit this awful situation for public sympathy.
As pp said, he appears to have been trying to get access. The youngest one didnt have his name, but the name of another man. Its likely he didnr have PR or she was denying the youngest was his.
It's not actually that difficult to block access for 18 months. Especially, if you are denying the child is the fathers and keep moving. Which she also appears to have done.
I agree with pp, assuming the father didnt do enough when the mother is that evil that she would murder her children, rather than assuming she is the reason for loss of contact, is very odd behaviour and appears to people trying to shift blame for the murderer because she is a woman.