What Harris is arguing is that, regardless of what I feel or what your hypothetical fascist feels about this problem, there is a definite, objective answer to this question, not to be found by comparing opinions, but by empirically examining nature and human society.

I should have been more clear about this. In my previous post, I'm supposing that killing the children would be the wrong thing to do, as I strongly believe our current understanding of human psychology and sociology indicates. But I could be wrong, and whether I'm right or wrong about this moral decision is not a matter of opinion, but is fundamentally a scientific question. This, as I understand it, is Harris's point.