I am not exactly sure *how* my kids figured it out but the first inkling I got was when they noticed the size of feet on Perla and Suzy mice at 1900 Park Fare... my older two dd's were 5 and 6 at the time and were talking about that. Then a few days later that same trip my dd5 (at the time) said to dd6 (at the time), "He's not really Mickey Mouse, but we should still be nice to him because he thinks he's Mickey Mouse."
I pretended not to hear. It did kind of make me feel bad but my kids are super observant and I'm pretty sure that they figured out the princesses don't necessarily look the same from one trip's pictures to the next. They definitely play along with it though and even though they "know" they are "characters" they enjoy them for the magic that they are.
My oldest dd, now 9, struggled all Christmas season to ask me if Santa was real. She'd try to bring up the question and stop herself mid-sentence... as if she wasn't sure she really wanted to know. Thankfully I picked up on it so I could prepare myself. I believe in the magic that characters and tradition and storytelling can create but I don't want to flat-out lie to her and eventually have her discover the truth only to feel foolish for trusting me. Christmas Eve she took me aside and asked me so I explained the true story/legend of St. Nick and how Santa is based on that. I was sooo mad when my parents told me the truth I was so full of anguish to endure the same hurt in my dd's eyes, but she was surpisingly fine with it. I also told her that as long as she keeps it to herself (if her sibs start asking questions she should say I don't know why don't you go ask mom, etc. -- I still have a 5, 6 and 8 yo and think 5 is pretty young to lose Santa) -- I would let her play Santa with me. She LOVED, LOVED, LOVED that! And even still, there were some family gifts from Santa that she didn't set out and she didn't know what we got her so she still had plenty of magic on Christmas Day... but for her, this year, the chance to play Santa was the most magical, mischeivous memory for her... and despite the typical scuffles with her sibs since then, she's protected the tradition of Santa.
I do think each child handles this kind of thing differently depending on their personality but this has been what's worked best for my kids. I won't lie to them but I don't really broach the topic... for the most part I think they believe in Disney, they are "characters" and that means there are people bringing those characters to life -- but the quality of the costumes and the interaction and the overall theming at the parks lends itself to being open to the characters as "real".
...what do you mean that there are people under the "costumes"
). I still believe in the magic and I hope that my girls do for a long time too! 
I think I scared him when I gasped and said they are not costumes who told you that. I felt really bad because he was upset. I think it was because I gasped so loud. I didn't mean to upset him.
I was shocked that he said that. My other children didn't say anything to him. They all like to enjoy the magic.
) I give him a great big hug back and tell him "You're my favorite!" 43 people! 
and Tigger is Tigger. 
