Every time we've gone, we've noticed about 30% wearing costumes. At the 100 Years show last month, we saw a mother and daughter (about 4yrs old) dressed as Snow White. Very cute.
My kids never dressed up because of the sitting.....too long to be in a costume. But there are plenty of kids who do.
The cutest thing I saw at the show last month was a group of 14/15yr old girls. They sat in front of us. They had Mickey tattoos, Disney shirts, Mickey ears, etc....they waved Mickey lights, sang songs and ate slushies out of HUGE princess cups. I loved seeing them have so much fun.