I didn't remember how long it lasted, but there are
YouTube videos from last year that are over an hour long. Looks like about 15 minutes of a preshow with singers, then 40 mins of Mannheim Steamroller, then more of the band and singers performing together the grinch songs.
Are they physically able to sit on the ground? We sat on the astroturf area in front of the stage, I don't recall any seating there, it's a big open space; there may be a few benches outside the stage area where they could sit and hear it but I bet they go fast.