A comment on your 2nd pair of images:
As you know, and have demonstrated, cartooning always involves is a simplification of the subject. However, it is almost always important to see the facial expressions in cartoons, so one should not simplify the scene by simply darkening the faces (or other large areas) so much that details such as facial expressions can no longer be seen.
There are practically an infinite number of ways to simplify a scene -- different styles. Below is just one example of an alternate treatment (more of a sketch than a cartoon) that preserves visibility of the kids' faces while moving the original image a bit into the semi-realistic realm.
Like yours, mine is also nothing more than a simple mechanical treatment: You press a button and the computer changed the brightness and color of each pixel, but did absolutely nothing to the geometry. However, when most people think of cartoons, they also expect some geometric changes - distortion and simplification to make a caricature or stylized representation of the person / scene -- the sort of thing you would get if you quickly sketched the scene by hand. You may want to think about incorporating those sorts of changes, as well.
No matter what you do, the less mechanical the effect looks, the better. Of course, an oil painting, a sci-fi fantasy, or an old-time pen and ink lithograph-like version of the scene would all be less representational than the original, as well as non-mechanical, so if you want to stick with cartoon-like effects you have to stay within the boundaries of each genre.
There are some folks on this forum who have been professional cartoonists. I'm sure they will have many suggestions for you.
Best of luck,
Tom M
PS - I did make one non-automatic change: I changed the color of the shirt of the central figure from a bright red to white. I felt that being red, it completely dominated the scene, whereas I wanted viewers' eyes to go from face to face.