Bridge to Terabithia
(2007)

New girl Leslie meets poor boy Jesse.
They become friends and create an imaginary kingdom in the woods called Terabithia.
One day Jesse leaves Leslie to go on a trip to the city without asking her to come along.
She goes to Terabithia by herself and drowns in a creek. He blames himself for it.
He eventually comes to terms with Leslie’s death; and creates a bridge over the creek to Terabithia, so no one will fall in the creek again.
He then shares the magical kingdom with his younger sister. 

Thanks Johnson from Wisconsin!