This is what my son did. We could do nothing until the ride was finished. My DH was insistent that he wanted to handle it, but I was furious when all he did was give him a 'talking to' and nothing more.
My inclination was to go home straight away, so that this would be a lesson he would remember. His behaviour had potentially tragic consequences. My DH said it was 'a mistake on DS' part'. My argument is that however wide-eyed and innocent a 'mistake' it was, it was an oppurtunity to divert future disaster by driving the lesson home in ways that would be memorable to him. Even half an hour of not going on rides while his siblings did might have sufficed. I am furious!