You'll have a blast. We celebrated one DD's 4th and our DS's 6th birthdays at WDW on 2 different trips.
You can get the birthday button at City Hall in MK.
On the restaurant board there is tons of info about ordering a personalized cake (more costly). Many restaurants provide a free cupcake or a small basic cake can be ordered at the podium.
Now with the new promotion I believe there is a center piece you can request and pay for...but before that some restaurants would decorate the table with streamers and confetti if you let them know in advance that you are celebrating a birthday (Have it noted on all your special ADRs). At some meals the characters will give you a signed card or place mat.
Also, go to the front desk at your resort and request a birthday wake up call (Ours was Goofy).
As for decorating the room, there are companies you can pay to do this, but we just did it ourselves. (much cheaper)
Hope that helps! You'll have a great time!!