The Return of the Indian by Lynne Reid Banks
It’s been over a year since Omri discovered in The Indian in the Cupboard that, with the turn of a key, he could magically bring to life the three-inch-high Indian figure he placed inside his cupboard. Omri and his Indian, Little Bear, create a fantastic world together until one day, Omri realizes the terrible consequences if Little Bear ever got trapped in his "giant" world. Reluctantly, Omri sends the Indian back through the cupboard, giving his mother the magic key to wear around her neck so that he will never be tempted to bring Little Bear back to life.
But one year later, full of exciting news, Omri gives way to temptation when he finds that his mother has left the magic key lying on the bathroom sink.
A whole new series of adventures awaits Omri as he discovers that his Indian has been critically wounded during the French and Indian Wars and desperately needs Omri’s help.
Now, helplessly caught between his own life and his cupboard life of war and death, Omri must act decisively if he is to save Little Bear and his village from being completely destroyed. What began as a harmless game has tumed into a horrible nightmare, a nightmare in which Omri is irrevocably involved, and from which he may never escape.