Am I the only person to notice the word "voluntarily" (sic) ? Presumably he meant willingness of both sides. I haven't given the subject much thought, but it's certainly not in the same league as someone joining you in your bathtub against your will.
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.
beyond that, children really don't have a sexuality until at least puberty. There is no such thing as "voluntary" pedophilia.
Perhaps rms was talking about the legal definition (18) and not the clinical