Innocence takes many forms in Bridge to Terabithia. Jess's mom doesn't know how to use correct grammar, and she believes her daughter when Ellie tells her that her dad promised her some money. Leslie doesn't realize that not having a TV is the mark of an ostracized outsider. Jess doesn't know the "stories" of Moby Dick or Hamlet. But while some characters, like Ellie and Brenda, think they already know everything and are experienced, that's not really true. Because Jess acknowledges his limitations and what he doesn't know, he's able to fill in the gaps. He can appreciate the beautiful and the unusual, like the art in the museum. Most of all, he's able to appreciate Leslie.
The more obviously "experienced" characters in the book – Ellie, Brenda, and Janice – don't have as much maturity or intelligence as the supposedly "innocent" characters like Jess and Leslie.
Both Leslie and Jess's parents are equally innocent in different ways – Leslie's because they live in an abstract, artistic world and Jess's because they can't conceive of that world.