A piece of cardboard cut into a circle shape seems to work pretty well. You could use two bowls to trace out the shape or just draw the shape freehand.
I wrapped some twine around the cardboard form and used some tape on the back to secure it in a few places. If the string is too loose, you'll have a hard time getting those leaves to stay put. This is a good time to tie on a piece of string from the top to hang it with.
So, there's a pretty cheap crafting project for a kid's birthday party. I'm sure the tweens will come up with far more creative wreaths than mine. I wish I could be at the party!