I think it could be Bridge to Terabithia by Katherine Paterson. Bridge to Terabithia is the story of fifth grader Jess Aarons, who becomes friends with his new neighbor Leslie Burke after he loses a footrace to her at school. Leslie is a smart, talented, outgoing tomboy, and Jess thinks highly of her. He himself is an artistic boy who, in the beginning of the novel, is fearful, angry, and depressed. After meeting Leslie, Jess is transformed. He becomes courageous and learns to let go of his frustration.
Jesse (a.k.a. Jess) Aarons, the only boy in a family of five children, lives in rural southwest Virginia. His mother favors his sisters Brenda, Ellie, May Belle, and Joyce Ann, while his father works in Washington, D.C., and therefore spends little time with his children. May Belle, the second youngest sister, adores and admires Jesse. Leslie Burke is an only child who moves from Arlington, Virginia, to the same area as Jesse. Her parents, both writers, are wealthy.
Jess and Leslie soon become close friends. Jess shares his secret love of drawing with Leslie, and Leslie shares with Jess her love of fantasy stories. With this new friendship, the two children create an imaginary kingdom in the woods near their homes, accessible only by a rope swing over a creek. They name the kingdom Terabithia and declare themselves King and Queen, and they spend every day after school there. In Terabithia, they are able to face their real-world fears, such as that of the seventh grade bully Janice Avery.
Leslie gives Jess a drawing pad and a set of watercolors and a tube of paint as a Christmas gift, and Jess gives Leslie a dog whom she names Prince Terrien, or "P.T." for short. They consider P.T. to be the royal protector, Prince of Terabithia and, due to his puppyish antics, court jester.
Jesse has a crush on his young music teacher, Miss Edmunds. The central crisis occurs when Jesse accompanies Miss Edmunds to the National Gallery of Art in Washington, D.C., and Leslie goes to Terabithia alone. The rope breaks as she is swinging over the rain-swollen creek. Though a good swimmer, Leslie falls into the creek and drowns, possibly due to head injury. Jesse can overcome his grief only with the strength and courage that his friendship with Leslie had given him.
He attempts to deal with his grief by going back to Terabithia alone to make a memorial wreath for Leslie. During his ceremony, he hears a cry for help and finds May Belle caught in the midst of a fallen tree that she had been trying to use as a bridge across the creek. He helps her out of danger and rescues her.
Leslie's grief-stricken parents soon decide to leave the area. As Mr. and Mrs. Burke are leaving, Jesse asks to take some of their wooden planks from their back porch. They say he may have anything left in the house; thus permitted, he goes down to Terabithia to build a bridge. After he finishes the bridge, he takes May Belle over it and makes her the new Queen of Terabithia. He realizes that while no one could ever replace Leslie, Terabithia must have a Queen and Leslie would agree.
Hope this was the book you were looking for. - Cass