L.L.
My husband and his mother have issues (bio mom and 5th or 6th husband, I can't remember what number she is on). Anyways, he doesn't want either of them around the kids (his mother is a peice of work in and of herself. He has no real issue with the husband other than he doesn't want to intruduce our kids to someone who in all likely hood will not stick around long- based on his mother's track record). So point being, these types of situations can get complicated. Our deal within our marriage is that he deals with his family and I deal with mine.
Your boyfriend knows these people better than you and he knows the ins and outs of the situation. These are his kids. If he doesn't want these people around them, I would roll with his wishes in this matter. The next time his mother contacts you I would be polite, but honest - "Tom is not comfortable with your husband. If you would like to work out arrangments to see our family, please call Tom so that the two of you can work out something you are both comfortable with. I would prefer to not get involved in making these arrangements."
Tom's mother is a grown up - she made choices and she has to live with them. If she wants to do things for/with the kids dispite the fact that her son does not want her husband around, she is making that decision knowing the facts. It is not like Tom is hiding his animocity towards the husband.
Refuse to be the middle man and let the chips fall where they may. When your kids ask questions, just tell them that sometimes adults have conflict and it can take time to work things out. When their Dad has worked out his issues with his mother, things might change. Until then, we have to be patient because we can't work this out for them. - or something to that effect. THe reality is that your kids will witness interfamily conflict (and hopefully resolution) no matter what, so make it a learning experience and show your kids that beingthe middle man does not help anyone.
Good luck!!!! This stuff is SO STRESSFUL!!!!!