I may put in a bag of M&M's but what I like to fill my DS's (age 7) Easter basket with is toys (ie, yoyo, yu-gi-ho cards) backpack clipon, pencils, notepads, & one or two Game Boy Advance Games. This year I may even throw in a book of MacDonald's Gift Certificates. He has gotten rolled up tee-shirts, bathing suit & a Playstation game in the past.
I would think for a little princess, it would be much easier (& more fun) to fill an Easter Basket with little girl make-up stuff & pretty hair accessories, maybe one of those 3 inch dolls (can't remember the name but they're real popular right now), there are also those mini Bratt Dolls too. (Too young for Bratt dolls?) You can put a puzzle in there too.
As for GoofieRuthie's question about whether to bring Easter Basket stuff along to WDW or wait till they get home.....in 2000, we went for Easter & I brought along a gift bag (with jelly beans on it) since baskets don't travel as well, & how do you hide them? I also brought along all the things I was going to stuff it with (sunglasses, candy, pez, pokemon cards, yoyo, Mickey Cap, & I can't remember what else...) and that morning while he was still asleep, I asembled it & set it on the table in our room. He was so excited to see that the Easter Bunny was able to find him. He was 4 at the time.