(One American family goes bankrupt and 20 Malawian children who otherwise would have don't die in childhood? On the face of it that looks like a pretty positive net outcome.)

If we're talking about donations on the scale of the activities that went into the mortgage crisis, I think you'd start to suffer seriously diminishing returns.

Even if you didn't, there are other problems you'd run into, such as the limited ability of the Malawian (or other impoverished African) society and economy to accommodate such a sudden spike in children surviving to adulthood. The lives that you save from extermination at the hands of malaria or other preventable causes are probably mostly going to be relatively lousy or short due to other causes, pending much further investment.