The more I read, the more I wonder if Dostoevsky's notion of a "positively beautiful man" is taken directly out of his experience with children.
The first time I came across a passage that made me think of Myshkin as a child was at Ganya's house when Nastasya Filippovna showed up surprising everyone. In that scene, Myshkin is the only one who sees some good in her. Like a child he has a pure vision, "And you're not even ashamed! You can't be the way you pretended to be just now. It's not possible!" (Dostoevsky 117) This response to Nastasya Filippovna's actions is so naive, but so true that it is almost indecent. That is a child, a child doesn't follow social norms, they cross them, they offend people, but they cross and offend from a good place. They have a clean heart, and so they don't know that the things they're saying will have the effect they usually do.
Going back, I also noticed that he was very childlike, because only a child could remember how a child's mind functioned. "Nothing should be concealed from children on the pretext that they're little and it's too early for them to know. ... At first he kept shaking his head and wondering how it was that with me the children understood everything and with him almost nothing, ... , I told him that neither one of us would teach them anything, but they might still teach us." (Dostoevsky 67)
The Prince is also portrayed as a sheep. Rogozhin says, ""You'll be ashamed, Ganka, to have offended such a ... sheep!" (He was unable to find any other word.)" (Dostoevsky 117). Perhaps in the ellipsis, Rogozhin paused to think of a better word to express the Prince's innocence. And he landed upon sheep, what a funny word. Sheep and sheepherding, common symbols in culture and religion, play important parts in Abrahamic religions. Is Myshkin the Lamb of God? Or is sheep used by Rogozhin to reveal Myshkin's childish nature? Which one, or is it both?