Hi, Diana! I like to mix up character meals from year to year too. One more personal disclosure: Ariel is my favorite.
You stand a good chance of seeing Cinderella and Ariel at the Princess Storybook Dining at Akershus in Epcot where you already have your dinner reservation. In the event you go 0-2 at the experience, you can catch Ariel at her new character greeting spot in Adventureland at the Magic Kingdom.
For Cinderella, she's a shoe-in at
Cinderella's Happily Ever After Dinner at 1900 Park Fare at the Grand Floridian. She's joined by her beaux, Prince Charming, along with Lady Tremaine, Anastasia and Drizella.
You may also be able to catch Cindy and Ariel at the
Toontown Hall of Fame in the Magic Kingdom. Make sure to visit when Toontown opens or late at night to minimize your wait.
(Did I mention I have two boys?)