Howdy Phyllis!  Both sets of grandparents were on my daughter's first trip to Walt Disney World and it was magical! 

Be sure to pack her stroller or plan to rent one when you arrive and take a towel with you to the Parks each day and use it to cover the stroller.  The stroller gets hot and will burn sensitive skin.

My mother-in-law gave my daughter small toys when we sat down for each of our table service meals.  Each time it was something new and fun and helped pass the time as we waited for our meals.

My parents also brought small surprises they presented each morning.  Once again, it gave her something to do while we were getting ready for the day.

Prior to being parents, we enjoyed virtually every attraction in the Parks and were there from open to close.  You will go at a very different pace this trip.  We now spend most of our time standing in line waiting to meet characters and our days in the Parks are much shorter.  Our 4 year old suggests Dumbo and Small World at Magic Kingdom, Toy Story Mania at Disney's Hollywood Studios,  Festival of the Lion King at Animal Kingdom and the Kidcot Fun Stops at Epcot.

I think the most important suggestion that I can give you is to plan but be prepared to be flexible.  Cherish each moment and be prepared for those experiences that will take your breath away.  Be sure you have your camera and take plenty of photos.  Everyone you are traveling with, from your tiny princess to both sets of grandparents will remember her first trip to Walt Disney World forever.  You are creating cherished memories that will last a lifetime and I am so excited for you!  Enjoy your magical vacation and let me know if you would like some other ideas to add some pixie dust to your daughter's first trip to Walt Disney World.