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.