I find it unlikely that the Chinese would open to hundreds of millions of young Africans immigrants to replace their children.
They don't have to. They are setting up schools, hospitals and training in Africa. They plan on moving a lot of their production there I assume, which is what the west did with China.
If that's their plan how to achieve geopolitical dominance, I'd say it could become difficult to implement. Energy from the Middle East, production in Africa, and consumption in China. No blue water supremacy and surrounded by a chain of major islands inhabited by people deeply suspicious of them or by traditional enemies.