There are lots of different answers to the “How should I live?” question: Have rational rules; look after your own interests; maximize happiness/good in the world.
One answer is “be virtuous”. This isn’t the same as just saying “be good”, since every answer to the “How should I live?” question can reasonably be offered as a way of being good. Being virtuous, having virtues, means something a bit more specific.
A virtue, according to Aristotle, is a quality that helps a thing perform its function well. He identifies the highest human good as eudaimonia, which some people translate as “happiness” and others as “flourishing”. In order to achieve eudaimonia, he thought, we need to develop helpful traits that will aid us in our function. These, like the sharpness of a knife aids in its slicing function, will help human beings in their function: exercising their rational capacities.
For Aristotle the virtues were all wrapped up in allowing the rational social human being to flourish. Some virtues, like magnanimity or truthfulness, are obviously socially beneficial; others, like courage are less obviously socially beneficial, but without them we will judge situations badly. The cowardly person will judge situations to be more dangerous than a truly rational person would see them to be, so being courageous helps ensure rational decision making.
Teaching kids how to be good people might mean giving them rules to follow, principles to adhere to, or calculations to make. But for the virtue ethicist it means enhancing their character with a set of virtues and trusting that being good people their decisions will be also be good, rather than prescribing decision-making methods ahead of time.