It's about the annual Christmas Pageant and the Herdman kids, a group of rowdy, dirty, and hungry kids who participate.
The Herdman kids start attending church because they hear that refreshments are served, and they end up with all of the major roles in the Christmas play (mostly because they bullied everyone else into not volunteering). They've never heard the Christmas story before, and the church congregation thinks that the Herdman kids will ruin the whole play, so there is a record turn-out. The kids don't end up ruining the play. Instead, they bring an awe and authenticity to the story, which makes it the best Christmas pageant ever.
Reading this as an adult was different from reading it as a kid. Reading the Christmas story from the perspective of kids who have never heard the story makes it fresh and new, something that isn't so bad around this time of the year.