Parents often devote significant resources to caring for special needs children who are unlikely to grow into good providers.

All the more reason to have a large extended family. These children will grow into adults who continue to need extra support, and there's no reason for parents to support them on their own. The more siblings you have to help out the better.

From a selfish perspective, the correct decision isn't to have more children. It's to kill or disown the ones who not only won't repay your investment, but will actually compete with you for the return on your other investment in your other children.

It's also not totally obvious to me that children are a particularly good investment from a long-term wealth or even a guaranteed income perspective.

This is because you are thinking of wealth as money. For much of the population of the world, and increasingly so as you go back in time, wealth means enough food on the table, enough food in the root cellar to get you through the winter, and enough grain seed to replant + keep you alive a year or two if the crops fail + plus enough to plant again once the famine is over. As long as another set of hands increases productivity, another pair of hands is a good investment.

I have a pretty broad-minded view of wealth actually. If you're a New Guinea highlander you can invest in mokas. You can trade your neighbors for goats or land. You can accumulate social capital by being generous and well-liked. You can enter into partnerships with younger partners. Another set of hands is only a good investment if it offers nearly the best return for investment, which is a much higher hurdle than merely "increasing productivity." It would actually be enormously surprising if the best selfish return you could possibly get for your time and effort was finding a mate and having children, especially given the high infant/child mortality rates. If children were such a good investment then why did we need a modest proposal?