Consider some imaginary website, run by a US legal entity - let's call it Whack Stove Reflow - has registered users, some of whom are under 18. This imaginary website wants to take a community satisfaction survey. Would it be illegal for the site to ask its sub-18-yro participants to take the survey, or to not-prevent them from participating?
Also, same question, but if the survey containing optional questions regarding personal details such as gender and racial background.
PS - If this may be illegal, please try to characterize the conditions/thresholds of legality.