My oldest was 6 when she leaned over to my mom while at the MK and whispered "grandma, don't tell anyone, but I think there is a real person inside of Mickey Mouse", LOL! She swore my mom to secrecy because she didn't want to ruin it for ME, LOL!
There isn't much you can do if they don't believe and honestly, it doesn't matter. There are so many magical things at Disney that you just need to find it in a different way. As my kids have grown, what they enjoy at Disney has changed, but each trip is still special and magical in its own way.
Pin trading, finding hidden mickeys, trivia about the parks, etc, are fun for all ages. You can also show her that it is still fun to "play along" with the characters even if you know they aren't real. Collect autographs, collect pressed pennies, point out the small details that make Disney what it is.