Demonstrate the importance of washing their hands properly by using UV Glow Gel
UV Lamp - (i.e. nail lamp)
glitterbug Lotion - http://www.handinspection.co.uk
As Scouts arrive for the evening give them all a drop of the glow lotion to rub on to their hands - not giving them any further instructions.
During or towards the end of the evening, encourage handwashing perhaps before offering them biscuits.
Get out the UV lamp and see who has washed their hands well, or at all!