I seem to recall there being a reference to this topic in some of the Sacred Band unit descriptions.

Anyway, I don't see why the Carthies could not have done it, especially if they were desperate. Religions can get seriously peculiar, and sacrificing your own children kind of makes sense in an "ultimate sacrifice" sort of way - I don't think there's ever been a culture on the planet that did not value its offspring very highly, and the whole point of sacrifice is to offer up something of value isn't it ?