L.D.
I would think that if the possible sitters are girls from your church, they are somewhat known quanitites. Even if you do not know these girls personally, someone in your church probably does and can probably vouch for the type of character they have and how responsible they are. As such, I doubt that an extensive interview and background check would be necessary. I think that just seeing how they are with the kids and what kind of vibe you get from her can really tell you a lot.
When I babysat when I was younger, nobody ever interviewed me or asked for my background information. Of course, I did talk to the parents and they did ask me stuff about what I've been up to and how's school's going, so that's probably their way of sizing me up. With my own babysitters, I've pretty much have done the same thing. All of our babysitters have been fantastic. I'm sure your's will be too.