When my DD was 2, she wore her Cinderella dress morning to night every day for a week at Disney. We begged her to wear something else as that dress could practically walk by itself. We got her in a Minnie costume one day, but she was absolutely not going to wear regular clothes. It started out because she went to a Princess meal in her dress, and all the CMs, and some of the park guests gave her a bunch of attention. "Hello Princess" "How are you today Princess" "Cinderella, you look beautiful". She was hooked. Our attitude was that she is at WDW, so if she wants to wear a Princess dress for a week, let her. Can't really do that anywhere else. Now that she is 5, she mostly prefers to wear regular dresses to the parks (She is a girly girl who insists on dresses rather than shorts, pants, or jeans. She would throw an absolute fit if I tried to make her wear shorts or pants at WDW). Sometimes she doesn't even wear a Princess dress to a Princess meal. Sometimes she will still want to wear the Princess dress to the meal, and then she usually does want to keep it on the rest of the day. She still begs to do the BBB, although I just hate how the girls look when they leave there. It is Disney, so we generally let her, but I would prefer not to. There are definitely little girls out there who want to wear those dresses all day in the parks. I'm sure there are some who have Mom deck them out too though.
As far as little boys, we always see several dressed as Buzz, Woody, or Pirates running around. At DL last summer we saw a Peter Pan for the first time. He was adorable. If your boys want to dress up, they certainly can!