Oh, my, I am so sorry you have to hear that. Especially from your Mom who should be there for you.
You have to remember, they are brainwashed and in a cult. They are trying to get you and your children back into the cult. Please be strong for your children, I am sure your children and going through a hard time.
When my parents spilt, I wanted so bad to go with my Dad, he was the one who left the JWs and we got stuck with our JW Mom. I couldn't imagine a more terrible fate than being stuck with a JW parent. As long as your children know that you love them and they don't have to be JWs, I am sure that they will see that there is light. As long as you don't diss their dad either. Try to let them have outside friends so they also see that Worldly people are not evil etc.
As for you EX: When he starts, I would tell him you are going to record what he says to play back to the Judge so you can have full custody. Or, just do it and then tell him you did. Call your attorney and ask what the rules are in your state. Don't let him browbeat you this way. You are not a bad person. You left a CULT. It is not a religion.
Also, get Steve Hassan's book--Combatting Cult Mind Control. He also has a web site which is very good. Type his name in and you'll find all kinds of info and helpful stuff.
If you haven't, try to find counseling in your area. It is also helpful. Family counseling for you and your kids could be long term help, you and they will appreciate down the road.
I wish your Mom was a better Mother. However, the Cult has it's tentacles in very tightly.
Hugs, VIII