One more vote for Animal Kingdom!! I went for the first time at age 9, and it was my favorite park then and it still is! (And I loved the countries at Epcot as a kid too, but as others have said, Animal Kingdom just has so much more going on in my opinion! Especially for kiddos!) My favorite ride is Expedition Everest for sure, Flights of Passage, Kali River Rapids, Dinosaur and it’s tough to be a bug are all fun, and the safari attraction is AWESOME! Both the Festival of the Lion King and Finding Nemo: The Musical are broadway quality productions, and those are great for a hot afternoon to get out of the sun and into some A/C! (If I had to pick just one, Lion king wins!) The younger kids especially will probably enjoy the Dino Dig play area, but honestly the 11 year old could have fun there too!
On top of all that, they have all the awesome animal viewing areas and trails, great food, and they recently added an Animation Experience at Conservation Station where you can learn to draw Disney characters (makes an awesome free souvenir!) we haven’t done this yet (we don’t have kids), but we’ve heard great things about the Wilderness Explorers thing (I think it’s like a scavenger hunt of sorts) .
Also, the theming is SO immersive parkwide. When you walk into Pandora, Africa, or Asia, you don’t feel like you are in Orlando anymore!
I’m a little biased towards AK if you can’t tell!

whatever you choose, have a wonderful time!!