A little while ago I came across a product called squid soap (nb. the description on the first site is not entirely accurate). To get soap to wash your hands, you press the squid’s head, which stamps you with an inky squid.
It seems backward to try and encourage kids to wash their hands by making them dirtier but, from what I have observed of child psychology, I’m sure it could work. Actually, after my recent trip to the dentist I am thinking of getting some disclosing tablets myself, which will highlight all the plaque left in my mouth after brushing.
I’m not so attracted to want to buy squid soap for myself, but it seems like a good idea if you are trying to train someone to wash their hands well.