Great to get other's perspectives. It helps me temper my reaction.
Obviously I'm not comfortable with it. I think we'll skip until he's older. I don't know of anything "drop off" for the under 5 year old set unless they are in preschool. I've never run into any classes where this happens. Around here at that age parent participation is often required as I've experienced. Or they sit on the sidelines and watch from afar.
As a former special ed preschool teacher I never saw special needs therapists exclude the parents from therapy. The goal was to teach the parents how to continue the learning away from therapy. So it was all geared as much for the parent as for the child.
My son does get instruction and has developed relationships outside the family. He happily participates in Karate twice a week. I get to observe from the side of the room. He happily goes to Sunday school every week with teachers I have personal ongoing friendships with and I'd be welcome to sit in at any time, every time if I wanted. He is also involved in basketball where I can sit on the sidelines and watch every practice and game.
I think I'd be ok with a COOP because it would be ongoing and I'd get to know all of the adults involved. If I couldn't well then I'd have to rethink it. We don't have coops near me. This museum is a situation where I'd have to walk him into a space he's never seen, with a room full of children he's never met and pretty much won't ever see again, to listen to a teacher he's never laid eyes.
And, well, my kid isn't in kindergarten in part because I'm not on board with being excluded from my children's life and learning. Especially from adults I know nothing about.
Leaving my child doesn't give me a breather either. It's a children's museum. There is no space for adults. Parents stand in the hallway or sit on the stairs until their children come out. It isn't like I can really kick back with a book or anything. You could go to another part of the museum with younger kids. But there is no way for the teacher to contact a parent if there were an issue. There is no sign in or out process. Children are just released from the room to anyone or no one waiting outside.
I do agree that parents can be a huge distraction. We are often rude and chit chat and distract. I've lived that as a former teacher myself. So I guess it works for others but it doesn't work for us. Maybe when he's older.