This is a spiritual sacrifice, where believers or Christians refrain from food for a number of hours during the day. This was given to men by God as an act of devotion to him. During this time one also engages in prayer. The Jews, on orders of God, fasted as much as three full days and nights in a row. But mostly from sunset to sunset they fasted.
One does fasting, refraining from food and fruit juices, soft drinks, coffee, tea etc., drinking only water
In the west with the sun rising as early as 4am and going down as late as 11pm, I regard it sufficient to vast from 6am to 6pm. This is twelve hours on water. There are many churches who take the soft line on the matter claiming that small quantities of food may be eaten during fasting. This is false, and not fasting! No food, sweets, juices or soft drinks, but only water is to be consumed during fasting. To ease the pains in your stomach, one hour before you start your 12-hour fasting, eat a meal and drink until you are satisfied. I can here assure you that this is sufficient to God – fasting for twelve hours and prayers, for in Israel the fasting period was also about twelve hours. Start with prayer, continue with prayers, if you can, during the day and end with prayer.
And please, do not force your children to fast; you shall only torment the little creatures and provoke God’s wrath upon you
because the essence of fasting is the sincere motivation of the faster to abstain from foods, for God. They are only children now, and one day they shall choose to fast through their own will, it’s just like with the baptism. Just give them a good example and teach them, making them read the Gospel: and in this latter, you may ‘kindly’ impose your authority upon them, because you love them. This is never to be done with the ‘iron hand’ or force.