In "We Are Seven," as in many of his poems, Wordsworth presents children as possessing a kind of primal wisdom which they gradually lose on growing up.
In his arrogance, the speaker naturally assumes that the little girl has no real understanding of death. After all, she talks about her dead siblings as if they were still alive. Yet as the speaker probes more deeply, it becomes apparent that the little girl knows a lot more about death than he originally thought. Whereas the speaker becomes ever more exasperated by the girl's constant insistence that "we are seven", she herself remains composed throughout, serene in the knowledge that her departed siblings are as present to her now as they were when they were alive.
The little cottage girl's superior wisdom is intimately related to her simple, childish faith in the benevolence of God. She is sure that the Almighty will provide her dead brother and sister with paradise in eternity to compensate for all the privation they endured in this life. It is clear that Wordsworth laments the loss of such an attitude among adults and that it is only by attempting to reconnect with the wisdom of childhood that we can begin to deal more effectively with grief and personal loss.