Me, broaching the subject carefully: "Have you ever heard the argument that you shouldn't stop a child from stimming?"
Person: "Yes, if the child is just doing it to entertain themselves, but if the child is stimming due to anxiety or stress, that's non-functional."
Me: "...but, dealing with anxiety and stress IS a function."
If I thought there was even the tiniest chance of them listening to me, though, I would totally have some long conversations with people about stimming, using a child's special interests, squishes, and the power of positive reinforcement. (And yes, that child can tell if your reaction is genuine or not.) But I'm just blown off every time.