Complex sentence: The child finally came to Snow, who explained that he would have to survive on normal air, then, sadly knowing that the child could not stay, sent him out into the world, telling him to find Bigby.
Split into simpler sentences: The child finally came to Snow, who explained that he would have to survive on normal air. Knowing that the child could not stay, Snow sadly sent him out into the world with instructions to find Bigby.