Hi, Lorin!
If I were bringing little ones the ages of your children for just one day in Walt Disney World, I would take them to
Magic Kingdom Park.
This is the classic, most iconic park in WDW, with so many attractions and entertainment options that are specifically geared toward families with little ones. From the moment you step onto Main Street, U.S.A., you'll know that you are in the most magical, special place, and your children will be swept up in all of the activity, too. While there are some attractions which they may not be big enough to enjoy just yet, there is enough to keep you all busy, and happy, for at least two days. (And that's just scratching the surface!)
Take a look at this great post entitled
Little Ones at Disney Theme Parks for more information about making the most of your day at WDW, Lorin. It's packed with information and tips that will really help you learn what you need to know for the best day at WDW, including details that pertain your young family. (Love these tips!)
Keep in mind, too, that you'll want to check park hours on the day that you're planning to visit WDW. Very often, the park with the longest hours is the best place to get the biggest bang for your buck. And the park that most often has the longest operating hours (not including Extra Magic Hours, which is for WDW Resort Guests) is Magic Kingdom.
Send more questions as you think of them, Lorin! We love to help!