By "child investments", they mean investments in children, not children-as-investments. In other words, they're suggesting that not only are children in these

By "child investments", they mean investments in children, not children-as-investments. In other words, they're suggesting that not only are children in these disadvantaged situations starting off in a worse place, but each additional dollar or hour of time spent on them gives less benefit than that same dollar or hour as a child from a more privileged background, further widening the spread between these two kids.