Hi Milagros,
It sounds like you are planning a great vacation! Since all children are unique, different attractions will appeal to each child. I always suggest that guests order the free
Walt Disney World Planning DVD. If you watch it with the children, you should start to get an idea of what they will like. That said, I can offer you some general ideas for each theme park and the ages of the children in your party.
At Magic Kingdom, if any of the children are enamored by the princesses, be sure to see Enchanted Tales with Belle for a retelling of the Beauty and the Beast story in which guests can participate. If the children are at least 35 inches tall, they may be ready for a first roller coaster, The Barnstormer. If your oldest nephew enjoys that and is ready for more, head on over to Big Thunder Mountain Railroad, for which one needs to be 40 inches tall to ride.
At Disney's Hollywood Studio, an attraction that appeals to all ages is Toy Story Midway Mania, on which you ride and play games simultaneously. Younger children also enjoy Disney Junior - Live on Stage, in which children are encouraged to sing and dance along to a show featuring Disney Junior characters. Finally, take in a show, like the amazing Beauty and the Beast - Live on Stage, a Broadway-style retelling of the classic tale.
At Disney's Animal Kingdom, Kilimanjaro Safaris is fun for all ages. You will board a safari vehicle and ride through the African savanna with opportunities to see animals such as lions and giraffes. Also, be sure to take in the amazing show Finding Nemo - The Musical, featuring wonderful music and larger-than-life puppetry.
I would love to help you plan further Milagros; however, I am limited in the amount of space that I have to answer. Please do not hesitate to ask if you have any further questions!