If the child is too short, even by a hair they won't let them on. And you have to be careful, because if it is close, on several occasions, on different rides, we made it past the first marker, only to be rechecked and not make it past the second. So even if they make it by the first one, don't get them too excited as they might not make the second one. This happened to us on Soaring, Gadget's and Goofy's Sky School.
I think they factor in shoe height when making the measurements, as DD I know is one height from going to the dr. and won't make it there for that height. Main thing is don't get their hopes up, it's so said to see them excited and then turned down.
Another thing, when we went with DD when she was 3 1/2 she was just a hair short of riding the 40" rides, so we made kinda a game of it, whenever she would see a height measurement she would get all excited and say "let's see if I'm tall enough for this one". She had fun doing it, and wasn't too bummed she couldn't ride it, other than jumping jellyfish, (of all things) haha. And when we went back the next year she would do the same thing and now she was tall enough for most, so she was even more excited that she grew
Have fun!