Hi Hillary,
The theater for Beauty and the Beast seats a lot of people. Although, you may not get a seat front and center if you arrive late, there is usually ample seating to accommodate all guests who want to see the show. However, I'm not sure when you are taking your vacation. If it's during a time where the park is a full capacity, then it may not be as easy to get into a show at the last minute.
Likewise, we've always had luck with the Disney Junior, Live. The seating for that show is on the floor, and the room is quite large. These shows are much shorter than Beauty and the Beast, so even if you didn't make it in for one show, the wait wouldn't be too long.
Now, Fantasmic! is a different story. This show does tend to fill up. While grandma could potentially save seats, I wouldn't advise going in at separate times. She may get in and save seats, and you may still be turned away because the stadium is at capacity. Your best bet is to all go in together. To shorten your wait, you may want to try the
Fantasmic! Dining Experience. This package gives you preferred seating for Fantasmic! when you dine at select restaurants in Hollywood Studios.
Hope this helps. Have a magical vacation!