It's not a dress. It's a kimono

I liked it alot transitioning from child to true monster is hard to right about and capture in words. You chose to bury the reader in the smells and unconscious needs of the budding bloodlust that will ultimately control the Slenderman to some extent. I would throw in the Slenderman's own beating heart, just to add more senses to the environment you are building.  Really good stuff!