Most rec kids do not have the endurance nor the desire to do 6.5 hours of gymnastics. We do a morning only camp (3 hours) and by the time they leave, the kids are just done. We do about 30 minutes at the beginning for stretching/warm-up, a 15 minute break in the middle, and about 15 minutes of games at the end. If we were going to keep them all day, I can't imagine doing any more gymnastics than that. I think we would probably do gymnastics rotations from 9-12, a break for lunch, and then spend the afternoon doing a craft, games, maybe some outside activities if you have an outdoor area, and a mid-afternoon break. You could bring them back into the gym at the end for some open gym time, but I can't imagine they would want to do much in the way of structured training at that point.
3 kids for 8 hours is rough, while a small group of kids might seem more manageable, keeping that number of kids engaged for that length of time is hard. Especially if the kids are not particularly interested. The girls are taking more turns, getting more worn out, and really can only go so long. Plus they are rec kids, they probably don't eat, sleep, and breathe gymnastics like a team kid does. We have a minimum number of kids needed to run a camp- to make it financially successful for the gym but also to keep it fun for the kids. 3 kids is essentially a really long private lesson, or a babysitting session.
I think you would be hard pressed to find a full day gymnastics day camp that provides a large quantity of gymnastics instruction, so I sort of doubt the coach in charge did anything wrong. And kids get out of camp what they want to get out of it. We had a handful of kids who did not learn much in the way of new skills over the course of the week, and that was with pretty focused gymnastics instruction. It wasn't for lack of trying on the part of the coaches, it was because the kids just wanted to hang out with their friends and do what they wanted to do.
I would respond to the email, apologizing to the parent and offering a discount. Tell them you would take their feedback into account for future years. Ask the coach who was in charge to make notes about what went well and what she thinks should be done differently for next year, also try to get an account of how the week went from her. Use all of this (as well as looking into other gym day camps) to start coming up with a plan for next year.