Kory:
Choosing between
Animal Kingdom and
Disney's Hollywood Studios simply comes down to whether you and your children spend a lot of time outdoors and love animals or are more into pop culture and watch the Disney Channel/Mickey Mouse Clubhouse, allowing you to be more familiar with many of its characters like Doc McStuffins and Jake the Neverland Pirate.
Because my children weren't able to meet the height requirements of many of the major attractions (Star Tours, Rock 'n' Roller Coaster, and Tower of Terror) at Disney's Hollywood Studios when they were four (look here for the
list), we spent most of our time at DHS at the shows like Beauty and the Beast, Muppets in 3D, The Little Mermaid, and Playhouse Disney. If your children are familiar with those Disney movies and characters, it would be a solid choice. Our family can't get enough fireworks and that's a big plus for this park as Animal Kingdom closes earlier than the other parks and does not include a fireworks display in deference to the animals.
I found, however, that when my children were pre-schoolers, they were just as enamored with the animals at Animal Kingdom on the Kilimanjaro Safaris and the gorilla trail nearby as any character from television they'd encounter at DHS. Animal Kingdom's Nemo-The Musical and The Lion King didn't need any previous references like DHS's Playhouse Disney. Animal Kingdom is a very large park, so I'd recommend a stroller for both of your children if you were to choose it.
Both parks have much to offer, just vastly different surroundings. You won't be disappointed with either choice.
Wishing y'all a magical vacation!
Whitney