I have never thought the posture could be different later in the day. I measured him at bedtime tonight. So crossing fingers his tall enough. If not he is used to one of us doing something else. This is his 3rd trip so one of us just does something else fun with him while we take my 7 year old on things he cant go on.
It's not necessarily the posture (though being tired can make you slouch), but the actual fluid that makes up the intervertebral discs. Those are the squishy little things in between each vertebra; there is liquid in there, and the liquid gets squashed out (not all the way) as gravity works on us throughout the day, and as stressors squish us down. Stressors like, say, rollercoasters or log rides.
So keep the kids well hydrated with WATER (no soda, no junk, just water! it's the best for us), keep them well rested! Getting in that solid 8-12 hours on the back/side (12 = my son, even still...if he gets that much sleep he's awesome the next day) helps mitigate the forces of gravity on our upright bodies.
Then go to the most wanted ride of the day FIRST thing! Make sure they stand straight and tall and help them understand that they WANT to hit that measuring stick. Hitting it means they get to ride! Slouching down out of concern of hurting their heads will cause them to not ride it.
And ultimately, if such a little dude can't make it or can't make himself not slouch to hit it, it's OK. When it happened to us (not his fault but a lousy CM, but I don't want to digress) and he couldn't ride Star Tours, ultimately we came to realize that a little one who is not mature enough to realize that doing this (hitting the stick) means doing that (riding the fun ride) probably isn't mature enough to deal with the ride.
Or worse, being on the ride, having it stop, and having to listen carefully to the CMs and follow mom/dad out of the ride, seeing backstage, etc etc.
So it'll be OK. Either way.