Legally, minors are considered "incompetent", the idea being that they wouldn't know if something would be harmful to them in the long run. Additionally, the level of wisdom/intelligence is just too different -- children are extremely easy to manipulate compared to adults, especially since adults are almost unanimously regarded as "authority figures", doubly so if a family member. Voluntary means little if you can't reason at the level of an adult. I don't think we can honestly say that a 7 - 10 year old child has the same reasoning and experience to make good choices.