I am very interested in this too.
I have a just turned 4 year old son (OK, he turns 4 tomorrow) but he is already 43.5" at last measurement.
We are going down again in December & he really wants to try Mission Space. He is really into rockets & space.
I am torn on whether or not to take him on there.
He is a 'daredevil' & has gone on every ride in Disney that he is tall enough for, even Tower of Terror & has done perfectly fine.
I think I will go on it first & just see how the ride actually is, but then again I didn't think he would be able to handle ToT but after he kept begging me & begging me, let I him ride it & he was fine. (although I was almost expecting him to freak out) so I guess you never know.
I would like to hear from others who have taken their younger child on this & how they did.