I'm so sorry you're going through this. Having to cope with shock, disgust, sadness and having to hold it together for your family.
It's important to sort it all out into different points.
He's a hebephile / ephebophile (see article).
You don't want him near your children.
This girl, whatever you may understandably feel about her and her parents, is his victim.
Write down everything you know and can remember about the fact that he began a sexual relationship with a mentally ill adolescent, and abandoned you and his children. This may help when contesting access / custody attempts.
Find out everything about your child support rights and go for what your children are entitled to.
His mother is their grandmother - there's a sad post here from someone whose mother cut off all contact with father's family and she feels sad about that to this day.
She will be grieving the potential loss of beloved grandchildren, and if they have a great relationship it would be a shame to end that.
Yes, she brought him up. There is that.
She will - hopefully - be apalled by what he's done, but he's her son and she will be possibly minimising and excusing him, so be careful what kind of messaging the children night be getting.
You'll want to be wary and probably only allow supervised access, maybe in a neutral public space.
All the best to you and your family.
https://www.psychologytoday.com/us/basics/hebephilia