I would book it as soon as you know what day you want it done. You could wait until you get there, but then you risk not getting any appointment at all.
By the way I saw that a little boy about 3 years old, got his hair done while he was waiting for his sister. He got a mickey on the back of his head and they spiked his hair and put other stuff in his hair and made it different colors. i don't know how much extra that way but they do it for little boys too.
Hope you have fun!