First, it is the duty of parents to inculcate the love of Islamic acts of worship in the hearts and minds of children. With simple introductory ways that suit the age of the young, we can establish the concept of fasting and many other Islamic concepts in our younger generations. There are many Prophetic hadiths to that effect. The Prophet (peace and blessings be upon him) is reported to have said, “Tell your children to pray when they are seven years old and discipline them if they don’t when they are ten years old” (Al-Bukhari).
Sheikh Muhammad Nur Abdullah, President of the ISNA (Islamic Society of North America) and member of the Fiqh Council of North America, states: The best way is talk to them first about the blessing and importance of the month of Ramadan. There are many hadiths on this subject that you can use. Encourage them to fast using examples of the Companions of the Prophet, and talk about the great reward Allah grants to fasting people. It is reported in a hadith that the Companions used to make toys for the children to keep them busy while fasting so they wouldn’t think about food.
So, you should train them to fast regularly. Even with little children who are still under age, they can start by fasting a few hours, to half a day, until they can tolerate fasting the whole day.