In my experience they are very strict with the height requirement.
The CM will measure the child before he/she can enter the line. They will ask the child to remove hats and measure to the head so big ponytails don't help.
Sometimes kids naturally shy away from the stick, tell them they want to stand straight hit it! If the childs head dosen't touch the stick they will not be allowed to ride, no exceptions. Unlike a poster prior to me I actually have seen a CM ask children to remove obviously high shoes (once sandles and twice sneakers)!
If a child looks close to the height a CM will likely measure the child again prior to boarding so be prepared for that as they could still be turned away.
I have seen kids get by the first CM (intentionally or not) only to be measured or re-measured at boarding and not allowed to ride
Honestly if I were the OP I would not plan on your dd's riding the 40" rides and then if they do measure tall enough consider it a bonus.
TJ