Ideas on how to make 2D animations look more gritty and dark

Everything Mylenium plus "Distraction".  Consider projecting the animations onto a cave / stone wall or make it part of a 3D scene or possibly pages in a book.  Basically give your audience a secondary focus, then any "for children" vibes will come across as a deliberate decision.

Hatching instead of solid fills. Gritty & grungy typically means some noisy pattern changing from frame to frame. So if you can somehow extract the shadow regions and use that as a track matte for a stretched out Fractal Noise at an angle and quickly cycling, this could massively make it more sinister. That and of course your images have no real depth. The color shifts as objects get farther away are unrealistic because there is no haze and the initial source colors are too pastell-y anyway. Some harsh color corrections and scrunching up the contrast would certainly help, too. If you have access to the original layered files, also get rid of those awful dark mountains in teh second picture. That's not how you draw such stuff and it just looks flat.

Mylenium