If I understood correctly the OP is raising a point I too have wondered about. I have also heard of people who had many ill children each of whom required very expensive treatment which they could not afford without external help (ie Tzedoka) but seemingly continued to have many further children anyway.
To my mind this boils down to the Psak of the Rov re family planning. Here in the UK at least one Rov advocates waiting until the mother is emotionally capable of caring for her baby. Some women need more time than others. Other Rabonim, especially in print (seforim re Niddah), are much stricter, and seemingly forbid family planning in any shape or form. In such a structure a couple has little choice but to continue procreating, despite the tragedies that ensue. I wonder if it would help if people in such situations were made aware that other Halachic solutions exist and that there are rabonim who are more Meikel. It certainly seems tragic and cruel, in some ways, to make a couple continually bring sick children into this world to suffer.