Oof! Horrifying! Thanks for the story!

You should also check our @suesa's page. They're writing a zombie outbreak story from the point of view of a chemist/biologist.

Criticism? I think the story is clear. I'd suggest a proofreading pass if you're submitting it for professional consideration. You could also take a pacing pass: i.e. look for extraneous and/or imprecise words ("She tried to sneak a peek at them." becomes "She sneaked a peek at them." Like Yoda says, "do or do not, there is no try" [take that with a grain of salt]) and cut or replace them and look for the moments that you want us to live in, and expand on your details there. I want to know more about the crack in the wall that she knows she can fit through but her folks can't.