Hi Katrina,
First of all, happy early birthday to your niece from all of us here at the Disney Parks Moms Panel! She is going to have the time of her life celebrating her birthday at the most magical place on earth!
My daughter's middle name is Marie, so Marie from Aristocats is one of her favorites, too! We also saw her in Magic Kingdom this past June. Character greetings can change daily and so there is no real way to guarantee that she'll be there when you go. If you don't see her there, it is possible that she might be at Epcot in the France Pavilion. I have seen her there many times in the past, too.
My biggest suggestion would be to check the park times guides daily for listings and times of the different character meets. Another useful tool would be to download the
My Disney Experience mobile app if you have a smartphone. This official app has an option to look up characters in all four parks and find out the times and areas they will be greeting guests. Not only is it useful for that, but it can give you up to the minute wait times for attractions/shows as well as organize your plans and reservations.
May her dreams come true during your vacation!
~Allison L