I think it’s hard to get constructive feedback from kids-- too much conflicting social pressure. Are the most of the kids old enough to have smartphones? Or will parents with phones be there? What I’m thinking:
Short riding lesson where they get coaching on proper position, use of arena, safety, etc. Then break them up into groups that will allow decent spacing for a W-T-C “class”, where they’ll be asked to put those lessons to the test.
The riders are all paired up with one of the non-riders. They exchange phones so the non-rider can take a video of “their” rider, then switch roles and have another brief class. So, each rider will have a video of themselves on their own phone.
Take an unmounted break, where the kids all sit down for 10min and study their own video. Ask them to identify 3 things they think they did well, and 3 things that they’d like to improve. Might need to coach them a little on what kinds of things to look at. Did they keep a safe distance at all times? Heels down? Hands, seat, posture, transitions, etc.
Self-critique I think will feel more safe for the kids.