Hi Andrea,
1900 Park Fare is indeed quite near the Magic Kingdom. It is at the Grand Floridian Resort, which is just a short monorail ride away from the Magic Kingdom. I'd encourage you to consider selecting a morning when the Magic Kingdom does NOT have Extra Magic Hours in the morning. Then, make your dining reservation as early as possible at 1900 Park Fare, ideally when it first opens. By dining early, you should be able to eat prior to the park opening and arrive at the park not too long after it opens. I think you would still be able to have a wonderfully full day at the park.
As for which park to spend more time at is a tricky question because of personal preferences, but our children really enjoyed Disney's Hollywood Studios more at the same age. It may help you to look over the
Magical Beginnings Guide that features attractions geared toward preschoolers.
Just a side note, you may want to check out www.touringplans.com, which is a site that offers over 100 touring plans to help you make the most of your days at the parks. Highlights the must-see attractions as well as the order to do them in. There is a small cost, but one I think worthwhile since you are planning an entire day at the Magic Kingdom.
Thank you for the question; please let us know if we can help again.