Hey Robin!
Congratulations on the new addition to your family!
The best Park for a one day visit that every one can enjoy is, I think, the Magic Kingdom.
The Magic Kingdom has that classic Disney World feel with all of the rides and Characters that you think of when you think "Disney". Your family, including that precious 2.5 year old grandbaby will have that emotional jolt of happiness when you stand at the entrance to Main Street USA and look up at Cinderella's Castle!
I think there is something for everyone there. My own little boy, who is now ten, had his first trip to Disney World at age 2. He still remembers a lot from the Magic Kingdom Park, but not really from the other Parks, even though he visited all of them that year!
I hope you have a great trip!