The whole reason we do experiments blind, have rules against nepotism, and worry about inadvertent bias is that we are humans and these things go with the species, no matter how fair we try to be. Even though there has been a ton of thought about these issues, they go awry. When I was in high school in Michigan I had some excellent teachers who should have been college professors but were not because of university-wide nepotism rules that negatively impacted women.
Another danger zone involves working with your colleague’s children. What if a high school student does so poorly in your group that you have to let them go? What if you do not have tenure and the parent of the child will be voting on you soon? What then? Will a parent take your side over that of his much-loved child? I suppose it will be easier to give the student busy work rather than antagonize the voting parent. But this isn’t fair to anyone either.
Over the years I have worked with many high school students in the laboratory. The less technical the work, the better. The more their project can be isolated from that of the group, the better, so if they mess up it does not compromise the work of others.
Perhaps our microbial work is more technical than our wasp work used to be, but now I am loath to take on high school students. They can be a burden to the grad students and postdocs that actually mentor them, just as undergraduates can be. But undergrads are much more likely to become productive members of the lab group and educating them is part of our responsibility and mission. There are a lot of other productive things high school students can do with their summers.
High school students in the lab seldom benefit the group sufficiently that they make up for the cost of training them, so I would be astonished to hear that anyone actually paid a high school student to join a lab group, unless it was an altruistic investment in a particularly gifted and needy student, or the tasks they were given were particularly repetitive and onerous. But repetitive tasks will hardly make someone fall in love with science.
So, resist the temptation to take on the child of a colleague. Resist the temptation to ask a colleague to take on your kid. The possibilities of unfairness are just too great.