Hi Silvia! Since Walt Disney World has four great parks to enjoy it is tough to decide which one to head to for a celebration day.
If you have park hopper passes you can consider being able to do both on the same day but if you want to just choose one it becomes a tough decision. Star Wars is a big part of
Hollywood Studios these days with much more to come. There are meet and greets, games, and displays of props in the new area where the Art of Animation once stood. Then you can venture over to the other side of the park and enjoy Star Tours which is a simulator taking you into the far reaches of space and beyond. Of course Hollywood Studios has other fun attractions such as Toy Story Mania, the Muppet Vision, the Great Movie Ride, and if your son meets the requirements there is Tower of Terror and Rock n Roller Coaster. Although this is quite a bit as there are a few more attractions in this park it actually could be a half day park depending on how quickly you move from place to place.
The
Magic Kingdom is definitely a full day of fun and magic and lots of pixie dust making this my choice for a celebration day for a six year old. Besides the multitude of attractions your son and family can enjoy, there are characters everywhere for meet and greets and to wish him a very Happy Birthday. On top of that you will find parades all day starting with the Move It, Shake It, Celebrate It Street Party held three times on Main Street. Then there is the 3pm day parade and of course the great fireworks in the evening. The castle shows and dancing in Tomorrowland make this a festive place to be. This would certainly be my choice for the most fun and entertaining birthday for a six year old!