Easiest way to figure it out is to focus on the most important part of the question. Why is the kids' relative living there? Assuming he/she is a zombiekiller like everyone else in the town, his/her reason for being there is the most significant, because it's also (transitively) the reason the kids are there, and they're the main characters. So once you get that part of the relationship sorted, the rest can just sorta fall into place probably -- and not everyone has to be there for all the same reasons, either, you could have like an "all of the above" answer. But yeah. The one that matters the most, is the one that matters to the protagonists.
I like choice 2, personally, but then again saving the world IS getting kinda worn.