Hi, Ivette!
Traveling with a darling little princess is so much fun, as long as you follow her lead! My best suggestion when
going to WDW with little ones is to try to keep on their schedule as much as possible. I know it is not always fun as grown-ups to be heading in for the night at 8:00 pm, but the more rest the kiddies get, the happier they will be! Try to keep not only with their nighttime sleep schedules, but also meal times and afternoon naps.
Make sure to travel with a comfortable stroller with a sunshade, and have a bag with everything you will need: diapers, juice boxes, snacks or crackers, water bottles, and a change of clothes. Have a hat, sunglasses, and sunscreen for everyone in your travel party.
There is SO much to see and do, you are right, but it is impossible to do it all. For each park, choose a few "must see" attractions or dining experiences. Here are some suggestions:
Magic Kingdom: Flying Dumbos, it's a small world, Many Adventures of Winnie the Pooh, and Peter Pan's Flight
Epcot: The Seas with Nemo and Friends, Journey Into Imagination
Hollywood Studios: Beauty and the Beast Live on Stage, Disney Junior Live on Stage, and Honey I Shrunk the Kids Movie Set
For dining, try making reservations for Crystal Palace, Chef Mickey's, and Akershus Royal Banquet Hall.
Keep your days simple, don't try to over-do it, and just enjoy the magic as it happens. Most importantly, take lots of pictures and have a wonderful vacation!