Hi Jose Ruben,
Walt Disney World is such a wonderful place for celebrations, and I am sure that you will have a great time celebrating your niece's first birthday there! My family and I have honored many birthdays at Walt Disney World. Let me offer you some ideas for this special occasion.
First, be sure to pick up a Happy Birthday button for your niece. These are available free of charge at Guest Relations at the theme parks, and at many other locations throughout Walt Disney World. You can also pick up buttons that say "I'm Celebrating" for the rest of your party, and indicate on them that you are celebrating your niece's birthday. It is really fun to hear cast members and other guests alike wishing your little one a happy birthday throughout the day! Although wearing the buttons does not guarantee any special treatment, you never know when a little extra pixie dust might be sprinkled on her day. My daughter received a huge birthday cupcake at The Plaza Restaurant at Magic Kingdom once when wearing a birthday button. I will never forget her huge smile at that moment!
You can also consider having a specialized birthday cake make for your niece. These can be delivered to most table service restaurants. Disney is amazing at customizing birthday cakes for guests! You will need to give Disney at least 72 business hours notice for this. Call 407-827-2253 to speak with a cast member about customizing a cake. You will also find that 6-inch chocolate or vanilla cakes are available at most table service restaurants with no notice needed.
There are many more options for
customizing a celebration at Walt Disney World, from special birthday Mickey Mouse ears to in-room celebrations. See the link I included for more ideas.
Have a great time planning your magical celebration, Jose Ruben!