We celebrated my son's 4th and 5th birthday at WDW. We stayed at Pop Century both times. Each visit, they sent balloons and a b-day card (signed by his favorite character) to our room. If you are eating at any of the restaurants on property, you can order a cake; however, many of the restaurants will bring a complimentary cupcake for the birthday guest. My son's favorite place to celebrate his birthday is at Chef Mickeys. On our last visit (a few weeks ago,) the server and manager at The Plaza brought out a beautifully decorated cake - we were all so surprised! I thought that was so nice of them. My son loved it!
Have a great time!