Actually, my experience is quite different from the other posters.
I have a very timid 6 1/2 year old and at every single ride she was tall enough to ride but didn't want to they would measure her and then tell me that since she is tall enough to ride, they do not issue the 'special' child swap Fastpass that they normally do for those who do not meet the height requirement but that we all had to enter the line together and then we could swap at the ride itself. This happened at ToT, Splash, Thunder and Space.
So, instead, what we do is get 4 Fastpasses for those rides and we just take turns with our older DD. DH rides with her and then I do. Works great!
Child swap passes are really meant for those who do NOT meet the height requirements. My advice is to ask each CM at the entrance to each ride you want to swap at and see how they handle it.
Have fun!