A watch dog is the answer to the children's joke, "What kind of dog keeps the best time?" Another funny joke for kids is, "What do you give a dog with a fever?" The answer is mustard, because it's the best thing for a hot dog.
Telling jokes is great for children. According to KidsHealth, a good sense of humor helps children be spontaneous, grasp unconventional ways of thinking, look beyond the surface of things, and embrace the playful aspects of life. Additionally, children with a well-developed sense of humor are happier, more optimistic and better able to deal with the adversities of childhood.