I would choose Disney Hollywood studios. One of the best shows at wdw is Fantasmic! A night time spectacular that brings Disney's The sorcerer's Apprentice to life with live action singing and dancing, fireworks, projections on water screens, and a heartwarming ending. Please look at this on youtube or another site, it is not worth missing. At hollywood studios there are alot more kid friendly things to do. I love Epcot but it wasn't until I was an adult that I started to appreciate the park. Epcot is divided into two lands if you will. FutureWorld and World Showcase. Future World has all the rides that the previous post mentioned, however world showcase has only 2 non-thrilling rides. World Showcase is beautiful. You can learn about different countries. However at Epcot there are no parades, minimal disney characters theme-ing (when compared to HS), and the night time show Illuminations fails in comparison to Fantasmic!
Only you know what your family will enjoy the most, so I encourage you to go to the disney website and see what each park has to offer. I leave with these points
EPCOT: 2x size of HS, Guaranteed meet will the core 6 ( Mickey and Pals), Awesome food, Great Learning Experience, BUT minimal ("rides"), and Disney theme-ing other than finding nemo, and Agent P (phineas and ferb)
Hollywood Studios: More child friendly, FANTASMIC!, Parades and random interactions with "stars", PIZZA PLANET, and more sit down broadway type shows BUT not as big as EPCOT.