So true, there is a tv show called "Fear thy Neighbor" based on true stories how so much violence escalates between neighbors over small incidents like barking dogs. Also, I wouldn't make suggestions to the neighbors over how to control the barking, they may feel insulted or take offense.
IMHO, Best to say how you hear the little dog barking when they leave and how the dog sounds distressed. Showing concern for their dog may win them over to take action. Coming across as a complaining neighbor may only annoy them and stop them from taking action.