Interesting if not an explosive topic for some. This is where playing PC, or at least somewhat fair, in your teaching conflicts with tact, or even the law. A confusing question that every instructor must answer regarding interaction between male and female students, or instructor-child situations.
I think most of the women on this and other forums would tell you to treat them like anyone else, but unfortunately, they are the minority when it comes to practitioners. Let's face it, at least in Uechi and the grappling arts I have seen, women are far and few between. And yes! That is a damn shame! But, let's look at each question you ask.
1. Girls under 13 I do allow to randori with the boys. In most cases anyway. It would probably depend, like anything else, how developed each participant is. Too strong is too strong no matter who we are talking about. The question is what happens if the boy graps or touches the wrong part of the girl's body. In my experience, I have found at this age, and in the middle of randori especially, they don't even notice. And this works both ways; even for the girls.
2.How do parents feel? Well, I ALWAYS have my doors open so that parents can watch the classes (good idea for all to follow). Without taking a poll, they seem to agree with the practice of allowing it, because when you enter in the fact that these girls have a better chance at being attacked by men in the real world rather than women, this is more realistic. Plus, I never let it go too far (that's the trick I think).
In the adult class, it really shouldn't be an issue as they are adults and they know what to expect in a free exercise such as randori. But once again, it is always closely watched by one or more.
3. What difference does age mean so long as there is no secrets to the training. If I am showing a junior female (or a male for that matter) a technique, there is always
other people around. It is NEVER done behind close doors or if alone in the dojo. If it is a bother, get another junior to aid in showing the student. You simply coordinate the move from the side.
4. My female students always practice moves with male instructors because a)I don't have that many female adult students and b)This is what is real and they know it. They don't want to be babied and to be treated with kid gloves. They want to know if this stuff works.
Hope this helps Len,