Wow...how wonderful are you, taking in your neice like that??! That is awesome! And it's wonderful that you're taking them to WDW!
As far as princesses, I have a 5-year old, who has a virtual 'shrine' to the princesses in her bedroom. We are taking her to WDW in April (shhhh...it's a surprise!).
This is what we have planned:
Dinner @ 1900 Park Fare at the Grand Floridian - a character dinner that normally has Cinderella, Prince Charming, the mice, and the Fairy Godmother.
Breakfast @ Akershus in the Norway Pavilion - this is a Princess character meal that normally has Belle, Jasmine, Snow White, Sleeping Beauty, & Mary Poppins. Link:
Princess Storybook Breakfast
Breakfast at the Cinderella Castle in MK - This is a TOUGH rezzie to get, but it is supposed to be worth it...!
Wonderland Tea Party - the Grand Floridian offers this, where the kids dine with Alice, the Mad Hatter, and others. Link:
Wonderland Tea Party
My Disney Princess & Me Party - kinda' pricey, but supposedly well worth it, from what I've read.... Link:
My Disney Perfectly Princess Party
For your neice's birthday, you can have a princess call her. Link:
Princess Phone Call
As far as souviners, I buy alot of mine ahead of time at the
Disney Store, Target,
Walmart, and the Dollar Store. They are 'secretly' packed away and will make their appearance during the trip. This saves me a lot of $$$.
As far as collecting autographs, it's easiest to book character meals, where you get a meal, interaction, AND an autograph. If that isn't for you, you can pick up daily schedules at each park to see where the characters will be, and you can then stand in line to get an autograph & photo.
As far as Universal Studios, the Nickelodeon Studios are at the original Universal, which has SOME neat stuff; keep in mind that their rides & attractions are usually 'edgier' than WDW.
Yes, the Millionaire Show is still at MGM. He could even make it to the Hot Seat!
Weather should be good at that time of year...!
No problem asking questions - that's what we're all here for! To help each other get the best possible trip in the world AT the world!