M.K.
Despite the stickiness of the situation, it is really not relevant how "Grampa"'s children treat (or don't treat) your Mother in Law or Vice versa. Likewise, the kind of relationship Grandpa has with his other/ own grandson is irrelevvant. What needs to be decided before you explain anything is whaat kind of relationship you are going to allow your children to have with Grandpa and on what terms. After that is decided, then you will have a better idea of what to say to all parties. Your kids only need to know that sometimes grownups don't get along and they decide not to be married any more. They don't need to know who's to blame or what transgressions were made by whom, or who was mean, etc. Then, they need to know when they are going to see Gramps (or that they won't being seeing him). Similarly, Gramps needs to know what the terms of the relationship are and what is acceptable (such as cards are OK, but no gifts for the kids or something like that). It isn't going to be easy, but it doesn't have to get ugly either. Best of luck to you in this difficult time.