Sobreviviente by Warlioness [mass effect flash big bang 2014]
10-year-old Justus Belleron and a handful of other children are caught outside of Cipritine on a school trip when the Reapers arrive and struggle to survive in this nightmarish new world. Many have died and hope is almost gone for those who remain. All they have left is each other.
(admin note: I will re-run this story here at tumblr once the art is in)