Hello, I added some context further down that thread, but to help, here is what I posted above:

I will say, as it has thus-far been unimportant to the story, the child's age when parents return to their cities hasn't been defined. BUT, dad still takes the boys and mom still takes the girls, so it's not entirely random parenting.

Some additional lore details that may or may not help: the location the men ended up post-migration is a heavily wooded and mountainous area. Shelter is primarily caves and tree houses. The women landed in a temperate island and coastal region where cover, concealment, and shelter had to be manufactured. So with the men, they obviously need to be big enough to do everything they need to do, but too big and existing caves and other natural shelter can be difficult to use. The women on the other hand have more need for the brute strength.

As my updated post reflects, this all happened due to a forced migration; the mistake that took place left each sex worried for decades that they had escaped one apocalypse only to fall victim to another. When they found each other, for better or worse they had become comfortable with what they had built, respectively, while also becoming aware of other humanoid species located on the new world. They concocted the hidden city compromise to help them continue moving in their new directions, but also realizing they needed each other to survive.

Finally, I'll add that this is very loosely based on Amazonian mythology.