The difference is Disney charges for the 3yr old, and not the 2. Not paying for park admission when you know you should is indeed stealing. Would you walk into a store and just take something small, because it was inexpensive, and expect it to be no big deal? After all, the store is not loosing very much money? Of course not. Yet for some reason it is ok to not pay for park admission, which is worth a few hundred dollars?
Oh, and to the poster who has 2 adjoining rooms booked, do you mean connecting? Because adjoining just means next to each other. Connecting means with the door in-between. Hopefully you put the right thing into your request, because if you meant connecting and said adjoining, that could be a big bummer.
And as far as a store honoring a mismarked price, that is not always the case. It depends on the cost of the item, and how far off it is marked. I had a fight with Meijers over that last year, and they would not budge. Besides that just sounds like a way to try and justify doing something that is not honest.