Let's assume the children in question are old enough to give meaningful consent - otherwise, I agree there is a problem. I don't think the decision to have your blood drawn is as impactful as consenting to sex - sex has psychological and physiological implications that are not considerations with getting blood drawn.

This isn't really exploitation, they understand what they give up, blood and time, and what they get - money. This seems more like a trade. Do you think that any trade between two parties where one has considerably more wealth than the other is exploitation?