As I dip my toes into looking at human subjects, it became clearer to me how hard it is to recruit subjects who not only fits the criteria you wish, but also those who would agree to be poked/prodded/electrocuted etc.
I got the link to this article in NYTimes from The Scientist' Community and when I started reading, I thought, "Hey, why not?" My cousin who was working on juvenile immune response recruited her nieces and nephews for her study. She bribed them to agreeing to come with her to the hospital to have their blood drawn. To date, there don't seem to be any kind of negative effect from their participation in her Master's project.
In general, children have an innate desire to please the adults in their life; be it a parental figure or any other caretaker. Is their consent for the study something taken for granted or is easily waived away because their parent(s) signed the parental consent form? What happens when they grew up and decided that they were not happy with having participated in the study and wishes for the data to be withdrawn? That is something that have been seen in adult participants who withdrew from studies.
I wonder if any of the children who participated in their parent(s)' studies have ever said something along the lines of, "Hey, my data helped you get that professorship. How about springing a car/Playstation/new dress etc for me?"