When is a child commanded to fast, and what is the basis for this ruling?

If a child has reached the age of discernment, his guardian must command him to perform acts of worship, obedience, and noble character. The age at which he should be commanded to perform acts of obedience is seven years, based on the Prophet's (peace be upon him) saying: "Command your children to pray when they are seven, and beat them for it when they are ten, and separate them in their beds." The Companions used to accustom their children to fasting even when they were young, as Al-Rubayyi' bint Mu'awwidh Al-Ansariyyah did with her children regarding the fast of Ashura.