We've all heard that one before. Of course there are children already unvaccinated by parental choice. Nothing unethical about studying kids who are already unvaccinated by parental choice. Hey, it would even be an opportunity to get one extra lecture in. I'm sure the ethical guidelines would require a lecture and offering all vaccines free of charge, should the parents change their minds.
Of course this wouldn't work for all types of studies, such as double blind placebo controlled, but it would work for some.