I've struggled with a tag line. For one poster, I've got the tag line "Growing up in Mott Haven has never been easy, and the apocalypse isn't helping". Since the book is set in Mott Haven and that particular store is in Mott Haven, that made sense: everyone there will know what "Mott Haven" is and why growing up there isn't a piece of cake. I don't think that will make much sense outside of that narrow environment.
I've played with a few others,
Variations on "It's just her, one ax, twelve kindergarteners, twelve garden stakes, and her will to live." That's pretty long, and gives me troubles. Works on the back cover, but doesn't seem to work here.
"Sometimes, you just can't hide": that worked as an intro to the back cover copy, but floating by itself didn't seem to work.
"God, Please make it so it ain't so scary down here when we wake up." That's a quote from the inside, but I'm afraid it would attract the wrong audience. While religion is certainly a theme of the book, it's dealt with as something the characters grapple with in an environment where the conclusion that any omnipotent creator hates them is inescapable. It's not an anti-religious book, but it certainly isn't a pro-religious book.
As always, I'm open to guidance.