Because fables are fairly simple stories, a good moral for a fable should be a simple lesson that a child can easily understand. Fables are great stories for children because they are usually quite short and involve animals as characters. The first recorded fables, written by the Greek author Aesop, include helpful moral lessons as part of the text, coming right at the end.
One fable with a strong moral involves a dog walking over a creek while carrying a bone in his mouth. The dog happens to glance down into the river and sees his reflection in the water. However, he does not realize that the water is reflecting his own image back to him. Instead, he thinks he sees another dog — with another bone. The distortion of the reflection makes the other dog's bone look bigger than his, so he decided to fight the other dog and take his bone. When he opens his mouth to start, though, his own bone drops out of his mouth and into the water. He can't recover his own bone, and he has lost the chance to take the other one too. The moral of the fable is not to be greedy and try to take what belongs to others — a moral from which kids in any time period can take a lesson.