My DS4 rode both EE and Space Mtn 6-8 times each during our last trip.
Our first day at AK, I just asked for a rider switch pass because I assumed DS was way too little (Pretty sure he was under 43" when we were at home). The CM said, "Nah, he's tall enough. I'll give you a pass if you want, but he can ride." We let him decide and he wanted to try it.
Of course my DD6 rode once and then said it was fun, but she didn't want to do it again because it made her a bit sick so she sat it out with grandma. She did the same thing with Space Mtn. But then on our last day, she decided to try it again and wound up riding 4 times in a row.
IMO, Everest is a more intense roller coaster, but Space Mtn gave me a bit more anxiety taking a small child on initially because you are in separate seats and can't see the child well enough to reassure them if they are frightened.