I think the dog and owner can be left as is- they are ubiquitous enough and it is the horse and owner which are actually the whole point. In fact, if I were going to do anything different, I'd carefully, narrowly, skinnily, outline the sign in that horse-pink; red draws the eye (first hue an infant sees is red) and it is the contrast between dog poop and horse poop that is being demonstrated.
This is GREAT work- this belongs in the Public Sphere somehow.