A few things that jump out at me right away are the ears, nose and the lighting on their chins. I would not recommend using a half disk shape for the ear, especially for smaller sprites like these. If you want you could try cutting the width down by a pixel and make it a little more rectangular. For the nose, I'm not really liking the shape. I would try make the nose a bit taller, cutting into the mustache a bit. I also think it would look a little nicer the nose shared it's outline with the mustache where it connects instead of having a separate brown outline. Lastly, the shading on their chins is a bit flat. I would having a light spot breaking up the shadow where the chin would be.
Try messing around a bit with the sprites to see what you like.
Try messing around a bit with the sprites to see what you like.