Hi, Kim! That's a tough question to answer since I don't know your girls or what they like, but I know if I was going to take my kids to Walt Disney World for just one day, it would be to the Magic Kingdom. It's my favorite park to begin with, so I know I'd want to be there more than anywhere else, but I also think that there is so much to do there, that you can find something that both girls will enjoy, and you can really put in a full day here and feel like you have accomplished a great deal.
If you'd like a better opinion, gather your girls around you and head on over to
www.disneyworld.com to let them browse the parks and attractions. Compile a list of things they really would like to see, and then you can all come to a decision about which park has the most must see attractions for them!
Enjoy your vacation, Kim!