Hi David,
Thank you for visiting the Disney Parks Moms Panel with your question. Preparing for your first trip to Walt Disney World is so exciting!
I personally believe that there is enough to do at all of the Walt Disney World theme parks to spend multiple days at each. Since you need to choose one to only spend a day at, I think that you should consider your family's interests. Often, when guests are choosing which theme park to spend less time at, they might choose Disney's Animal Kingdom or Disney's Hollywood Studios. If you have a family who really loves animals, it could be that you want to spend two days at Disney's Animal Kingdom. If your family really enjoys the shows and more thrilling attractions, you might want to spend two days at Disney's Hollywood Studios.
To help you make this choice, I have a couple of suggestions. Have your children make their own
customized maps of the four Walt Disney World theme parks. These are free of charge and can be delivered to your home before you leave for vacation. Your children can indicate on these maps which attractions they are interested in at each park. When you see the maps that your children make, you can see which theme park has the least attractions noted, and make that the park that you only visit for one day. Also, if you can, leave the last couple of days of your vacation with no theme park chosen ahead of time. As you explore the theme parks early in your trip, it will probably become apparent to you and your family which parks you would like to revisit. Plug those in for the last days of your vacation.
I hope that these ideas are helpful for you, David. I would love to help you plan your vacation further; however, I am limited in the space that I have to answer. Please do not hesitate to ask if you have any further questions!