We did this for our DS, now 3 1/2. He got a certificate, lots of stickers, they wrapped up some of his hair for us and he got Mickey ears w/ "first haircut" on the back.
We still take him every trip, it's tradition now. He loves going there and now that he's older, they put colored gel in his hair. He gets lots of attention, expecially from the characters.
Make sure you go very early! Even in the off season, the lines start quickly and the waits get long! The also tend to run out of mouse ears towards the end of the day for 1st haircuts. The day we went for his first vcut we got there at about 11:00am. My DS was the last one to get the mickey ears that day. The little girl after us wasn't able to get them.
They do not take reservations, it's first come, first served.