Like Chris L said, it's a futile pursuit. I used to try but more often than not I got a "F*** off kid, it's none of your business!" from the couples with no kids.
The people with kids are generally a bit more polite as they don't want their children to hear that kind of language.
Sometimes if I see a family riding without helmets I just wave then tap my helmet. I don't know if they are getting the message but I usually get some sort of reaction from them.