I don't think it had anything to do with rules of the road. I recently moved from the Seattle area to the Boston area. My experience is that if you want to avoid having profanities spewed at you for no reason, you should move somewhere away from Boston.
But to answer your question, I don't think that bicycle endorsements are practical. There's a precident that children are allowed to ride bicycles (otherwise, how would they get around? And how would bicycling start if not with kids?). And I, for one, would be disappointed telling my four-year-old that he couldn't ride his bike on our cul-du-sac without taking a written test
But communication of the rules does need to happen, and it should be communicated soon after kids start learning to ride. Perhaps some sort of communication in early schooling that could be re-enforced every once in a while during school would work. Then, later, re-enforced during driver's ed. Are there any videos aimed at young kids who are starting to ride on roads with other traffic?