I think they pretty much always choose a couple of adult men to be the "suits of armor." And warm up your operatic Joanne Worley vocalizing because this silly lady got to be the Wardrobe this year! "Mrs. Potts" was an older child when I was there, but I think she's sometimes an adult.
The suits of armor are usually adults and often reluctant fathers. I wanted to do it and made eye contact with our hostess when she asked for guards and I didn't even have to raise my hand. She called me out of the crowd as if I didn't have a choice. My plan worked like a charm.