Given the millions killed by malaria and at most thousands of experimental subjects, it takes a heavy thumb on the scales of this argument to make the utilitarian calculation come out against.

If it would result in a timely cure for malaria which would result in the disease's global eradication or near-eradication, I would say that it would be worth kidnapping a few thousand children. But not only would a world where you could get away with doing so differ from our own in some very significant ways, I honestly doubt that a few thousand captive test subjects constitute a decisive and currently limiting factor in the progress of the research.