It's shocking enough that this needs to be pointed out, even more shocking that someone would disagree with this statement.
1, The girls are not old enough to consent, so what they "know" doesn't matter and, 2, just because someone is mentally aware of what they are doing, doesn't mean they are in a position to make a rational choice that does not harm them. That's what the "age of consent" is for, because even if they're willing we say they are not allowed for their own protection.