Find one of the Skeleton Dogs; the easiest place to do so is Pumpkin Hill as Knuckles, and combine it with your Chao. As either Knuckles or Rouge, start digging around in the Chao Garden. Eventually a pumpkin will appear. If you give it to the Chao mixed with the Skeleton Dog it will put it on its head. If you want to take it off, either throw or hit your Chao.

You can find pumpkins in the regular and Hero Gardens, and skulls in the dark Garden. Get them by digging as Knuckles or Rouge.