Well, dd1 needed precise instructions along the lines of:
Get about 4 sheets of loo roll and fold them over
Stick your bottom out a wee bit
Try and wipe right inbetween your bum cheeks
Keep wiping with fresh loo roll until it comes away clean

Dd2 thankfully worked it out for herself but I always told both of them that if they weren't sure if they were clean or not I would check over their work 
However, even now at 9 and 6 you will hear me bellow...WASH YOUR HANDS!