Disney Don'ts: Follow These Rules and Your Visit Will Be Magical

Disney wants to make sure that when kids are interacting with characters, they are interacting with the right characters. So you can't just wander into a Disney Park in costume (although there are plenty of t-shirts featuring Disney characters that would be no problem at all).

Here's the rule under "Prohibited Activities": It's against the rules to be "engaging with other Guests or impeding the operation while posing as or portraying any character in costume."