I assume you're referring to Numbers 31.9. The verse says nothing about sex with children. Or are you talking about v. 18: "...but save for yourselves every girl who has never slept with a man." There is nothing in the phrase "save (keep alive) for yourselves..." implying sexual pleasure. It simply means, "Don't kill them, but let them live." You don't understand that in Israelite culture, all herem was considered to be the possession of God. Deuteronomy 21.10-14 makes clear that it was against the law for a Gentile female POW to be used as a sex object. Rape is excluded, and so also pedophilia.

No, the principles do not repudiate sex with children. Moses and the boys even took little girls as plunder in numbers 31.

No moral code or law code is able to give coverage for every potential situation or case. Law codes are designed with case examples that can be extrapolated to other similar conditions and even extenuating circumstances. Moral codes are more accurately defined by principles than by specific citation, for no list of citations can ever be complete enough to cover all possibilities. Exodus 21-23, for example, is known as "The Book of the Covenant." It contains casuistic law: examples from a variety of legal situations intended to form of basis of principles that could be applied to the situations not specifically listed. The laws anticipate a range of life situations without spelling them all out.

The point is that a person is not free to interpret to his heart's content. In the examples of the Bible, sex is between adults. In the narratives of the Bible, sex is between adults. In the sexual teachings of the Bible, sex is between adults. The Bible speaks often that childhood is a separated and protected stage of life:

Jesus' words in Matthew 18.6-10. "Allow the kids to come to me, for the kingdom of heaven belongs to such as these."
James 1.27: Orphan children should be cared for.
Deuteronomy 1.37-40 shows deference to children.
Numbers 14.29 uses the age of 20 to distinguish children from adults.

Even the Hebrew word for "children" includes persons from age birth - 20. We have far more to go by than "anyone is free to interpret...according to their own whim" and "all we're left with is societal convention."

The Bible has told us what is good: sex between adults committed to one another in a marriage relationship.

But the point is, without a specific age being given, anyone is free to interpret the idea that sex with children is bad according to their own whim. A person with pedophiliac tendencies could perceive that general concept, and think "well, clearly it's wrong to sleep with anyone who's less than four years old." It doesn't really help anything. Without the Bible spelling it out, all we're left with is societal convention and humans exercising their best judgment. From an atheist perspective, that's not a problem, because we believe that's where all morality ultimately stems from anyway. From a Christian perspective, it would seem like it is a problem. According to the Christians, morality comes from God, but what good is it if God doesn't actually tell us whether something is good or not?

Even without specific mention, it matters. Principles derived from regulations and practice count. The spirit of the law is often more important than the letter of the law.

> As I said, the principles of biblical teaching repudiate sex with children.

That may or may not be the case, but kinda the whole point of this thread is that it doesn't really matter, if the Bible never actually defines what "children" means.

I wouldn't say that at all. The Bible speaks deeply about sexual propriety and morality, it only speaks of acceptable sex in adult contexts, and there are no examples of marriage or sexual liaisons in the Bible with 12-year-olds that would make me agree with what you're trying to push here. As I said, the principles of biblical teaching repudiate sex with children.

So, Biblically, is it okay to sleep with a 12-year-old girl?

Notice that I said "It's fair to say." Of course disorders are possible, but given that menarche started later then than it does now. The average age even in our era is 12.43, and fewer than 10% of U.S. girls start menstruation before age 11. It is thought that even just a century ago the age of menarche was two years later than it is now. Who knows what the average was in the 7th century, but you're really pushing things to assume that she was fertile at age 9 in AD 625. So don't give me this "that certainly isn't the case." There's no "certainly" about it. It's fair to say she had not yet experienced menarche.

> Since 'Aisha was only 9 (though her age is disputed), it's fair to say she had not yet experienced menarche

This certainly isn't the case. Age 9 would be younger than most, but certainly not unprecedented or extremely rare. It probably would have been more rare back then because there is some evidence of the average age falling in modern times. But even so, there are disorders a person can have that would cause them to start menstruating at abnormally young ages (although age 9 would not necessarily require any disorder).