Because crayon doodles by children are both unrealistic and fantastic (in the unusual sense), while the professional renderings surpass (some of) the lack of realism.
Plus the artist took a few liberties adding their own touch, and said artist seems to have a dark flair.
The last one with the skull on a crucifix already had a dark flair even in the child's version. As one of the comments in the original article said, kids do have dark sides, whether or not adults are willing to admit it.
The lyrics to nursery rhymes are often pretty dark.
kids do have dark sides
I don't see anybody saying otherwise.
They're part of http://www.themonsterengine.com/ which asks children to draw a monster. The artist has an interview with the child about their drawing, and paints the child's drawing.
Because (A) the artist asked for monsters, and (B) a lot of kids draw with far more disturbing intent than many adults will acknowledge. Look at a lot of those original drawings and tell us they're NOT supposed to be disturbing ("chomping baseball" and "crucifiction" for starters).