"Let's imagine she's going to be sexually active with an adult; how would it be that the adult would approach her, and that this interaction, or relationship, would happen, and what would assure that everything that happened was consensual, and that she would be effectively protected from harm (given that her size and youthful naivete could make her particularly vulnerable)?

I've been to the Philippines, where kids aren't kept so carefully segregated from the adult population. You can walk right by their school playground, for instance, and there's no fence keeping you at a distance from them. They don't ride on a separate school bus, either; they ride the same bus that adults ride, so they might be sitting right next to an adult on their way home from school, without any parental supervision.

But, it seemed like parents kept a closer eye on their daughters and didn't let them run around as freely as the boys. I assume they figure, if their daughters could go wherever they wanted, they could end up getting snatched up and put in a cage in someone's basement, since they're a valuable sexual commodity. The typical Filipino dad would view it as his responsibility to keep his daughter undefiled until marriage."


Cage in a basement is likely not the main issue.
In a hut, you don´t have a basement. Neighbours in next huts would hear a girl in a cage, and act.
If you do have a basement, you likely live in a villa. Which would be full of your other servants. And they would gossip.
If you can afford a basement, you´d probably be better off keeping your bedmate pampered upstairs, so that your other servants can see and hear that she´s properly pampered, and report as much to her parents and envious neighbours - that even an unwed mistress of a rich man is better off than a wed wife of a poor man.

The issue is not so much someone snatching her up for his basement long term (as shown above, anyone who can afford a basement can probably afford to make a honest woman of her). The issue is someone getting her into bushes for a short time, and leaving her with a baby in her belly and/or Easy A on her name.
And the risk group for that is first of all boys her own age or only slightly older. Not so much rich adults/foreigners.

Also note that the boys her age or slightly older are also the most likely group to actually marry her. And those are related. Precisely because marriage is desirable and likely gives the boys with less honest intentions the credibility to get a chance to rape instead.