My son went twice (technically, although he was only in honor guard the first time) and my daughter went once. 2011 and 2015. While some of the details depend on the travel company you are with, here are some things your children may experience:
Marching Band: March in a parade around the Magic Kingdom
Concert Band/Orchestra or Jazz Band: Play at the bandshell in Disney Springs or Epcot.
Chorus: Sing at the bandshell in Disney Springs or Epcot.
Chorus: If going between Thanksgiving and Christmas. Sing in the Candlelight Processional!
Band & Chorus: Practice & Perform Disney music in a recording studio with Disney professionals.
Kids will likely get a pair of mouse ears:
And a drawstring bag:
And time in the parks, obviously.
Note that my children's school had a chorus and band that went on the trip. We do not have an orchestra, but we do have a concert band, jazz band, and marching band. I'm not sure if an orchestra gets different opportunities.
There were other things my kids did (a buffet at Rainforest Cafe, for example), but I think they were all specific to the touring company we booked through.
http://www.disneyyouth.com/our-programs/performing-arts/sp/performance/#categories-performance