Shui Naodai

Both are short anime about weird world around a kid. Both are dementia genre and can have a trippy feeling to it. While Water Brain has a story, Kodomo no Keijijougaku is just full of randomness. 

Both have the same feeling. I mean, both have the same dark and surreal environment. Also, there's no (not so) voice acting (I mean dialogues). So that means for me that both have the same feeling! 

Both are short films set in a city with (apparently) no adults, in which demons use children for the same general purpose; the design/feel of the cities - and to a lesser extent, the organic/machine hybridization of the monsters - is similar. Water Brain is much gentler and ends on a much more upbeat note.