at MK, you can have a meal at cinderellas royal table and she MAY be there. In toontown you can go to the tent with the princesees--i have never done that --but she could be there
At EPCOT--sometimes she is in germany. We have met her twice at Akershus as well
Is Magic Kingdom the only park you are going to?
She has also been seen in the Town Square Exhibition Hall there.
When at the park, you can ask a Cast Member if they know when and where she will be appearing.