what you have to ask yourself:
would you make your 8 year old daughter wear a helmet if she had to cycle 5 miles everyday to school and back?
And after you ask yourself this, you have to realize that for a great many things there are other rules for 8 year olds then there are for adults. In this case, however, members of the Dutch Cycling Master Race including Yours Truly generally think cycling helmets are also bull**** for them kiddies:
if yes, why dont you wear a helmet yourself? (suppose its a macho thing ....i.e. I'm rough and I'm tough and I don't need a helmet)
If you're going to answer your question yourself, why do you even bother to ask? Not that your answer is an accurate representation of the typical motivation for the choice to not wear a helmet...