you gotta be careful though...
I did a similar thing with my kids one night. We were doing the whole kung-fu thing and after a while I picked up a towel, rolled it up quickly, and then started to use it as a nunchuck. My oldest came in to whack me and as he retreated I flicked at his butt with the towel.
I didn't do it hard at all but it caught him just perfectly. There was an audible snap and we all sort of froze. I started to laugh but then noticed my boy was tearing up. He then showed me the welt....and it really was not good.
We don't do the kung fu stuff anymore.