In the end, it's far more important (in terms of safety, as well as for other social goals that cycling promotes) to get people on bikes than it is to micromanage the way they do it.

It's worth nothing that helmet usage is extremely low in pretty much all the great cycling nations (Netherlands, Denmark, Japan, etc), all of which have cycling mode-share orders of magnitude greater than any place in the U.S.

[In Japan it is becoming increasingly common to see very young kids wearing bicycle helmets, especially when they're seated on their parent's bike, but they seem to stop wearing them sometime in their grade-school years.]