My mother is convinced that my ex husband will be able to get money from her will. I told her if she writes in the will that he doesn't get anything then he doesn't get anything. I also have two children 12 and 17 and she thinks he can claim the money because of them. My opinion is what ever she writes decides who gets what.

You are correct. Furthermore, he doesn't stand to inherit anything whether she has a will.

Whether you children will inherit from her is another matter. If you fail to survive her, it is possible. But that would be their inheritance, not his.

Does he have custody?

If she is afraid that he would get the inheritance in behalf of the children, and perhaps mismanage it, she should set up a trust, to benefit the children, but with a trustee other than their father. She should pick someone she trusts to look out for her grandchildren.

You are correct, she should not have to list him, but it will not hurt to name him and that he gets nothing,

There is a way he gets his hands on the money. If something happens to you and he gets custody of the kids that could give him access to their funds.

So the answer is to set up a trust with a separate trustee that will manage the money until the kids are 21 when they get it.