If we had a similar upbringing would we think it morally correct to kill Yazidi children for having the "wrong" religion?

The question is irrelevant. If it is wrong to behead children for having the "wrong" religion, that is not affected by fictional scenarios in which "we" believed differently. (It's not clear what "we" actually means there, but that's a separate philosophical issue.) Truth is not found by first seeing what you believe, and then saying, "I believe this, therefore it is true."

Or might genetics play a role in our differing moral views?

This question is also irrelevant.

I find it hard to think of ISIS members as human

Well, they are. Start from there.