Symbolism, Imagery, Allegory
Listen up, because Shmoop's going to tell you something profound: The game is life. As kids, Zach, Alice, and Poppy aren't quite ready for the world yet, so they use the game as a stand-in. It gives them space to role-play different scenarios and work through their fears. So much so that when the Queen scares them in real life, "to avoid being totally terrified, Zach, Poppy, and Alice had added the doll to their game" (1.38). They simply can't work through their anxieties about this creepy doll without bringing her into the game.
Importantly, Zach and Alice, whose parents are (to varying degrees) absent, use the game to explore issues of abandonment. So as much as the game represents their lives, it also enable them to imagine different lives for themselves. Cool trick, right?
Despite the game's merits, though, it's still something childish. Zach, Alice, and Poppy play it instead of engaging in the real world, escaping in a way that's totally cool for kids but not-so-cool for older people. Fittingly, after the kids go on their quest in the real world, they don't need the game in the same way anymore. They've dealt with life head-on and are ready to keep doing so instead of mediating their experiences through the game. They are more adult now that they've made it on their own out and about in the world, which is made crystal clear by their move away from the game.
Worry not, though, the death of the game doesn't have to mean the death of the kids' friendships. "Maybe we can't play it the way we used to," Zach says, "but we could still tell each other what happens next" (16.120). Insofar as the game is a way for these three to bond, then, it looks like they'll keep on keeping on, no matter how mature they become.