In "it is a Beauteous Evening, Calm and Free," Wordsworth is walking with his young daughter one evening.
While walking, Wordsworth is filled with deep thoughts. Based on his words, however, it appears his daughter is not. He says, "If thou appear untouched by solemn thought" to indicate that, unlike himself, his daughter is not contemplating all sorts of deep thoughts while walking in nature.
However, he says that just because she is not engaged in deep thought, her "nature is not therefore less divine." He means that she is just as spiritual as he is even if she is not having spiritual thoughts about nature.
To summarize, when in nature, Wordsworth has deep, spiritual thoughts; his daughter, on the other hand, does not.