We go to this a lot. I haven't seen the benches in back fill up, but I also don't go at the busiest times.
If you are concerned, I think this is close enough to a medical accommodation that, if I were you, I'd ask the CM there (there is always one manning the fp entry at the show) if grandma could enter (alone) with fp people to get a seat on a bench. Tell them she just cannot sit on floor but is eager to see the show with dgd. They are very kind there to my special needs DS. I'm sure they can help out a grandma. Of course there could be a dozen grandmas that show who need the bench seating, but it's worth a try. (Or if you have a fp to burn that day/park, choose this show.). The first show of the day is usually less crowded so you might consider going then. HTH.