This is a way of talking about an imaginary situation.
Another, simpler way to express the same idea is this:
I'm a little worried about what would happen if we had an earthquake.
Adding "were to" makes the sentence sound less realistic. So in this sentence:
What would happen to the kids if we were to die in an accident or something?
...the speaker doesn't expect to get in an accident; it's just a hypothetical (imaginary) situation.