Do you hit the brakes, slide into the child and probably the truck too and kill everybody?

That's the most likely thing to happen. That kind of question masquerades as a question about morality, but it isn't, it's about psychology. In practice , in that situation you'd be acting on instinct, modified by the way you've been behaving all your life. (If the question was "Sitting in an archchair, what do you think would be the best outcome" it'd be completely different - and pretty irrelevant.)