Symbolism, Imagery, Allegory
When Others disguised as human soldiers take Sammy away to train as a child soldier, he leaves his beloved stuffed bear with his sister. For Sammy, the bear is a symbol of childhood. He has to leave it behind when he goes to the military base, where he'll be forced to do a lot of growing up. (And a lot of Crossfit-style exercising. Sammy must be the buffest five-year-old ever.)
For Cassie, the bear becomes a sort of personal mascot. It's a symbol of hope, of innocence, and of Sammy himself. It keeps her going in her darkest moments. When she's finally reunited with Sammy, she says he "slapped that damned bear out of my hand" (81.64).
He doesn't need it anymore. Not for nothing, Cassie refers to him as the more adult-sounding name "Sam" (instead of Sammy) throughout the rest of the book.