Hello Blake! Thanks for your question. I think that if every one of us here on the Moms Panel took a shot at answering this question, you'd get a variety of responses depending on individual preferences. With this in mind, I'm going to tell you which park I would choose for the second day and why. I would choose Disney's Animal Kingdom because not only is it my favorite park, but also because I think it would have the most for your children to enjoy. This can be argued, of course, but in my opinion, Animal Kingdom might better hold the interest of your children. Right now Disney's Hollywood Studios is kind of in an in-between state where it has closed down a few attractions to make way for the new
Star Wars Land and
Toy Story Land expansion coming in the next few years. Sure Hollywood Studios has Toy Story Midway Mania, but I would much rather spend my afternoon watching The Festival Of The Lion King, walking around and looking at all of the wildlife, and enjoy one of the amazing themed restaurants that Disney's Animal Kingdom has to offer. My family loves the atmosphere and menus of Tusker House and Yak & Yeti, and if you choose to dine at Tusker House, you will get to see Mickey, Donald & friends in their safari gear! Speaking of safari, the attraction Kilimanjaro Safaris is extremely child friendly, and one of my favorite attractions in Animal Kingdom. My son likes Epcot, but it doesn't hold as much appeal to him as Animal Kingdom does. There are certainly child friendly attractions at Epcot, and the World Showcase is an educational and fun thing to explore, so really it is up to you to decide which park to choose.
I hope this helps. If you need anything else please feel free to ask. Have a magical vacation!