Been there done that - our oldest finally turned 7 this year so she could ride alone if she needed to...our 2 and 5 year old each had a parent and grandma watched the baby!
So we visited last week - just as a word of caution, our 2 soon to be 3 year old was 40" by my measuring, at at the pediatrician a couple weeks prior. We stayed at the courtyard marriott and they wouldn't let him on the water slides because he was WAY short of 40" according to their measurement surfboard. He wasn't ever turned away at DL/DCA, but for a couple rides it was super close and if he had worn different shoes it might have been a different story. In particular, the measuring stick on the 2nd floor of GOTG seemed to be a hair taller than RSR for example, and bless that lady's heart, one time through the CM spent about 5 minutes getting him to stand the right way so that for a split second his head touched it. Nobody cared to measure on the second floor after that ride. Anyway, my point is, if you're right around 40" be mindful of the shoes he/she is wearing and also I would maybe not get hopes up entirely in case the 40" isn't there according to some CMs on some rides (it was a bit subjective on who cared to measure and how many times to remeasure....BTMRR always measured twice, one ride measured at 3 separate points but I can't remember which one.) Good luck!! I mentioned to a CM I wish they would have one general measurement and like add it to the ticket or something so we didn't have to waste 5 minutes at each ride getting my toddler to stand up straight, but it sounds like they tried that once and some people ruined it for everyone by cheating (per usual. ha)