During the matinee of Les Miserables the kids are there for the bow because clearly they have the evening preformance to do.
But, I know someone mentioned Billy Elliot and how it has three actors rotating the lead role. The thing is that on broadway there is an equity rule saying that they have to announce who is on and if there are any cast changes for the preformance your seeing. This includes child actors who are rotating a role. I can bet anything that when the show opens up on broadway there are people who will get a hold of their preformance schedule and that people who became hardcore fans of the show will only go on the particular day that their favorite Billy is preforming.
I am not sure about if kids get paid differently then adults because they are not doing the same number of preformences. If you want a good example of two adult preformers who get paid the same but don't do the 8 shows a week, take a look at the actresses playing Christine in Phantom. Neither one of them does all 8 shows. Yet, there is no difference in pay.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll