"Little Children" tells the story of a group of adults who feel trapped in their lives in middle-class American suburbia. The story takes place in a fictional town called Bellington, Massachusetts. Bellington is considered an quiet, idyllic town, but it is thrown off its axis when Ronnie James McGorvey, a convicted sex offender, moves into town. The story is told in past progressive tense, one character experience at a time.

Todd and Kathy are married with one son, Aaron. He is a stay-at-home dad and is supposed to be studying to take the bar exam for the third time; she is a documentary film maker who is working on a project at the local Veteran's Affairs (VA) hospital. Both adults are miserable in their lives, as Todd doesn't want to take the bar exam and Kathy doesn't want to be the working spouse, yet neither one of them seems to know how to talk that through with one another.

Sarah and Richard are married with one daughter, Lucy. Sarah was a literature major in college but stays at home with Lucy, and Richard is a brand consultant who is addicted to pornography. They've been married for less than four years, but both adults are miserable and looking for ways out of their marriage. Sarah turns to an extramarital affair with Todd, whom she meets at the school playground, and Richard turns to pornography.

Larry is married to Joanie and they have twins, but Larry's story is more about his fight with Ronnie, the convicted child molester who recently moved to town. Larry is a traumatized ex-cop who emotionally and cognitively cannot handle being on the police force. Yet, he fights for justice as a lay-citizen by trying to drive Ronnie out of town. His violent outbursts scare the town and upset his wife, who ultimately leaves him.

Ronnie is alienated from the entire town with the exception of his mother and her best friend Bertha. He goes on one date to satisfy his mother, but after that seems to accept the fact that he has no friends. After his mom dies, he cries on Sarah's shoulder and subsequently admits to killing a young girl several years earlier, a crime for which he was originally cleared.

May is Ronnie's mother. She is elderly and knows that she is ill, even though she hasn't been to a doctor any time in the recent past. She is upset by the town's treatment of Ronnie, especially in regards to Larry. Her response to Larry's treatment of Ronnie - including a lit bag of dog poop on her front step and neon orange graffiti on her driveway - is to support and love her son.

Mary Ann is married to Lewis, though they are minor characters in the novel. Mary Ann represents the judgmental parents who point out to others what they're doing wrong. Mary Ann works hard to keep up appearances of a perfect life, however, by the end of the novel her marriage is on its way to ending.

The adults in "Little Children" are the main characters, despite the title. They deal with unmet expectations, social expectations, feelings of loneliness, anger, and frustration, and work to eliminate feelings of boredom in their lives.