Family Jewels is slang for a man's penis and testicles, which are important for having a family and also for sex. The sperm is stored in the testicles! Also, getting kicked inthe family jewels hurts a lot!
"ouch, that soccer guy just took one in the johnson family jewels."
--also, every episode of america's funniest videos includes at least one guy getting whacked in the johnson family jewels by a kid with a baseball bat, golf club, a kid swinging at a pinata, or some other predictable manner