Ptolemy's chapter on children, in the fourth book, starts from the midheaven and the places that follow it rather than from a fifth house. He takes that region of the chart as the one concerned with offspring and examines the planets in it and their aspects, to establish whether there will be children, how many, and whether they will be reared.
The last of those is a category modern readers pass over too quickly. Ancient natal astrology treats the survival of a child through infancy as a question distinct from its birth, because in the world the technique was built for it was a distinct question. There is a separate chapter in the third book on children who are not reared. Reading these passages as though they were about family planning misses what they were for.
Valens and the wider Hellenistic corpus add a further layer through the lots, calculated points derived from the distance between two planets and projected from the ascendant. A lot of children belongs to that family of points, and where the house catalogue gives one fixed compartment, the lot method gives a movable marker that can fall anywhere in the chart.
The fifth house of children is therefore a medieval simplification of something the earlier tradition handled with a region of the chart, a lot, and a survival question kept separate from the birth. The simplification is what almost every modern textbook teaches, and it is worth knowing that it is one.
Sources TetrabiblosClaudius Ptolemy with Hellenistic Astrology: The Study of Fate and FortuneChris Brennan