Everyone's child is different, as are the parenting styles. What worked for us was having a folding stroller for our children. Many times they wanted to walk, but the stroller was there when they were tired or when I needed to put them in time out. (when they were walking it was a great place to stash my purse, otherwise known as the 'bottomless bag of holding' by the rest of the family).
We did have to put my little one in a time out at the Great Movie Ride once. DH just missed the ride while he stood in a corner and the rest of us went on, but we didn't take afternoon breaks (staying offsite), so he would have done better with a nap/break.
If you think your little one will be jealous of a ride her older brother gets, split up. Take her one way and let your son go with dad (or you) on the ride. Essentially, head off the tantrum before she discovers the 'injustice' of being left behind.
People say don't cater to your kids, but on vacation, you want things to go smoothly. It's not catering to your kids if it's maintaining a pleasant vacation for all. You can teach them lessons about fairness and sharing and patience when you're at home.
Finally, a huge majority of people who are also there will have kids now or have raised kids. They will sympathize with your plight if she has a meltdown. Don't worry about them. No one is judging you.