you can get the buttons at any park from guest assistance. they will write your kids name on the pin with marker, but if you are on the trip for a while the name rubs off so bring a prem. marker to re write the name.
we did it this past December for my middle child's almost 2nd birthday and it is SOOOOOOOOOOO worth getting. every CM, every character, every restaurant, every bus driver, every maid said something to him about his button and his birthday. when we were watching the parades they would walk by and say happy birthday.

he got special messages put in his autograph book. when you are at dinner and you tell your server they will bring you child a special cake and little place mate or pic with the cake. and if you are at a character meal they all come over and sing happy birthday. it was the coolest thing!!
but what was the best was when we were standing in the exit waiting to go on the teacups with the kids (my oldest DS 9 has CP and Autism) and a wonderful CM came over to talk to us and my kids. She then asked if we wanted to be riders of the day and announced over the Mic that it was both my middle one and my oldest son's UN-birthday and had the entire line sing to them happy UN-birthday. it was the coolest thing ever.
so yes get the button make sure your kid wears it were ever you go and you will really see how magical Disney is.