Glad to see this actually, as I have two non-joining-inners as well. DS was terrified of any organised activity. He'd either sit in frozen horror at such things as Baby Ballet or toddler sing-songs, or outright scream himself senseless. He was very sensitive to unpredictable events or change, and we had two investigations via the school for ASD. However, he was deemed not on the spectrum and just a little, um, fearful I guess. Or anxious. He is enjoying some drama classes at the moment, but is frightened of costumes.
Anyway, along comes DD, totally different kid, very social and eager to try new things. Here it is, I thought, time for some of that magical parenting where we go to Baby Yoga sessions, or pop on a ballet tutu, and get out and about! Yeah, no. DD is so stubborn that at the very hint that you WANT her to do something, she will refuse, even if it's something she likes. So an instruction like "let's sit down and sing!" meant a screaming tantrum of utter rage. She roared through ballet, curled into a ball and roared through yoga and, and this is probably my worst parenting memory ever, I ended up taking them BOTH to a kiddie soccer class, where she roared NO I WON'T at everything while my son curled up in a corner sobbing in fear.
Swimming with us, she enjoys. Swimming lesson? Screamed herself into a nosebleed. She spent her last lesson roaring in rage again, and her face went so blotchy they turned grey and I thought she'd bruised herself. She came out looking like an angry fruitcake.
We don't do classes now.