Wow. You have a mix of ages like me. Fun times, right?
Since your kids see you use magic but they also see their grandmother every day, as long as it’s her practices they are seeing, they should be fine. Them asking her about her beliefs should be fine, too.
I would have age appropriate discussions with each kid and remind them not everyone believes the same way. It’s natural to have curiosity about other beliefs and that you encourage that, even when they become adults they should always have questions instead of hard answers.
Then, like @MeganB said, it’s time for a sit down with your mom. You need to explain to her that you love her and you know she is doing her best at protecting her genetic line but she’s not to interfere with the natural development of their personal faiths. They can ask her questions and you encourage her to openly practice her faith like you do, but she isn’t allowed to undercut you or other beliefs. (If her god is so great, she has nothing to worry about anyway, but I wouldn’t say that to her. )
I like what the others have said, and obviously look at all of the responses, but I personally wouldn’t start taking my kids, especially the younger ones, to different religious buildings during services. Maybe tours during after hours? We do that in our city. The reason I would avoid the services, is I would want them to decide when it is time to investigate those further. Most places, at least around here, will be happy to do a tour of their facilities and discuss the faith in a general sense. Maybe avoid more hard-line/fringe things like Apostolic’s, Jehovah’s Witnesses, and Southern Baptists because you’ll never hear the end from them.
Again, no answers, just responses on this. Parenting is hard with no easy answers, but you know this already.