Hi, Andrea! Besides our name, we have a lot in common...you sound like a planner! First of all, great park choices for the ages of your children, and I think you'll guess my suggestion for a princess character meal -- Cinderella's Royal Table. However, if you can't secure a table at Cinderella Castle, then check out Cinderella's Happily Ever After Dinner at 1900 Park Fare (Grand Floridian) or their Supercalifraglistic Breakfast. The breakfast is not a "princess" meal, but my little girl was quite pleased with Mary Poppins and Alice in Wonderland.
In addition, I like the monorail resorts' early breakfasts because you can usually get in there earlier than the park ADRs, and you're not coaxing your kids through the park as you sprint to breakfast. Check out Lilo & Stitch at 'Ohana at the Polynesian, and don't forget Chef Mickey's at the Contemporary. Those are my boys' favorite character meals on the entire property...including my teenager son. (Shhh! That's a secret!) Also, consider Chef Mickey's for a late dinner after you're done at MK...it's the first stop on the monorail, and it's a great place to view fireworks.
At DHS, there really isn't a good character meal for your age children unless your younger two are still into Playhouse Disney. There's a breakfast at Hollywood & Vine featuring characters from their preschool programs. I would suggest going to the Cape May Cafe at The Beach Club and seeing Admiral Goofy and his crew. It's a bountiful buffet, and we've never been disappointed with that character meal. Afterward, take the boat ride over to DHS!
There's a wide disparity of prices between all of these character meals I've suggested, and you can find price ranges as well as make those valuable
ADRs by visiting the site or calling 407-WDW-DINE. Have a wonderful day and a magical vacation!