Our DS went on SM last November at 4 1/2 when he hit the height mark. The first time we tried to go, he didn't like the dark in the queue (is more afraid of dark than the speed factor of rides) and bailed at the very last minute so we ended up doing child swap. We talked about it during the trip how he was so excited before the trip to do that ride, and he decided to give it a 2nd try at the end of the trip after he had already mastered Mt Everest twice at AK. He did ride it, but it was NOT his favorite ride. We did not make him do it again, but now since time has passed, he has talked about doing it the next time we go, so apparently it didn't scar him for life or anything. I sat in front of him and put my arm back where he could hold my hand. That was physically rough on me, but reassuring for him.
I would say if you take your child on it be prepared to bail at the end of the line, or child swap or something if they change their mind. I hate to see parents "force" their kid on something they obviosly don't want to ride, so I would never do that, but some coaching sometimes works if they are just a bit hesitant. He did try Mission Space on the Less Intense version and that is his new FAVORITE ride besides Test Track and Soaring. We still haven't tackled Dinosaur at AK yet because of the dark thing - that one I'm letting him totally decide since it is a longer ride and very dark with things jumping out at you. His friend goes on it though and totally loves that one and they are the same age, so everyone is different. Have a great trip!!