Take possession of your stomach, before it takes possession of you. —St. John Climacus
Along with the need to observe whatever degree of fasting, parents must also take care to prevent their children from forming a habit of overeating, or eating too often, outside of the times established for taking food—eating between meals. St. Theophan the Recluse gives parents advice in this regard: “A child should eat in such a way that while developing and fortifying the body and giving it health, he does not foment flesh-pleasing in the soul. Regardless of how young your child is, he must begin from the earliest years to stabilize the flesh, which leans toward coarse matter, and accustom it to self control, so that in both the childhood and teenage years and beyond, he can easily and freely control this need.”
When the children grow and their personality and inclinations become clearer, parents should show tact with respect to the norms of fasting. They must not, for example, forbid them sweets against their will, or make the fast days so strict with regard to food quality and quantity as to exceed the norms of the Church’s rule of fasting. Ailing or frail children can naturally be given dispensations, or even be freed from fasting.
Young adults should also not be forced into a strict observation of all the norms of fasting if they are overburdened by them. Such forced fasting will not bring benefit to their souls, and could even harden them against it. The Lord said, I will have mercy, and not sacrifice (Mt. 9:13). The whole meaning of fasting lies in voluntary self-restraint and placing limits on yourself. Therefore, children should be taught to fast from an early age, so that the norms of fasting will not prove burdensome to them when they are young adults.
From How to Keep the Fast [in Russian].