If your difficult child is talking about the same type of Bloody Knuckles that I remember from childhood...
t's a challenge where you put your hand down flat on a surface such as a counter or table. Then someone hits you across the knuckles with something both hard and sharp, such as the edge of a deck of cards.
It hurts like H-E-double-hockey-sticks, but if you can sit through a round or two without flinching, wincing or worse, crying, then the mean ones tend to leave you alone.
Sorry your difficult child's having to endure this. Sending hugs to both of you.