That is horrible. I read that and got p!ssed off, angry! How dare OW and her do that to your sweet S9.
I understand you have called lawyer, GAL, police, and cps. I do hope one of these departments can find their way through the red tape maze and actually get something done!
I am sure you have, and are, reassuring the kids that none of this is their fault.
Press your lawyer, there must be a legal avenue to address this. Document this abuse and submit it to authority - L, GAL, etc...
Praying for you and your kids.
DnJ
Feelings are fleeting. Be better, not bitter. Love the person, forgive the sin.