Stephanie,
It is so hard to pick, but if forced to choose with a 9 year old and a 19 month old I would go for Animal Kingdom. The decision, for me, is more about having enough for the 19 month old to do. While selecting that park to keep the little one entertained, I don't think you have suddenly doomed the 9 year old to a day of boredom, I think they will love it too!
So for the 19 month old there is all the animal viewing, from
Kilimanjaro Safaris to all the exploration trails. The
Flights of Wonder Show as well as the playground areas of Dinoland USA should be good too.
Both will enjoy shows like the
Festival of the Lion King,
Finding Nemo-The Musical.
And the 9 year old might even be ready for more up-tempo rides like the
Kali River Rapids!
As a comparison at Epcot, I think that much of the World Showcase would be lost on the the 19-month old, as would much of the Future World side.
I am glad to answer any follow-up questions you may have. Just go to my bio page to ask!
Have a magical vacation!