Alhamdulillaah, here is a fatwaa to help parents help the youth who are fasting or are in training.
Intention to observe Sawm
Q 1: Should the intention to observe Sawm (Fast) in Ramadan be made at night, or is it permissible to make it during the daytime? To give an example, if someone is informed at forenoon that today is Ramadan, should they make up for the Sawm of that day later?
A: It is obligatory to make the intention to fast on any day of Ramadan in the preceding night before dawn. Thus, starting the Sawm in the daytime without making the intention in the preceding night does not discharge the obligation. Accordingly, if someone knows at forenoon that today is a Ramadan day and then makes the intention to fast, it is obligatory on them to abstain from all invalidators of Sawm until sunset. They also have to make up for the Sawm of that day later. This is based on the Hadith related by Ibn `Umar on the authority of Hafsah (may Allah be pleased with them both) that the Prophet ﷺ said: Anyone who has not made the intention to fast before Fajr no Sawm counts for them. (Related by Imam Ahmad, the Compilers of the Sunan [Imams Abu Dawud, Al-Tirmidhy, Al-Nasa’y and Ibn Majah], Ibn Khuzaymah, and Ibn Hibban. The latter two Imams judged it to be authentically transmitted in the Marfu` [a Hadith narrated from the Prophet ﷺ with a connected or disconnected chain of narration] form.) This ruling applies to the obligatory Sawm. As for voluntary Sawm, it is permissible to make the intention to fast at daytime, if no eating, drinking or sexual intercourse has taken place since dawn. This is because it is authentically reported on the authority of `Aishah (may Allah be pleased with her), that the Prophet ﷺ came to her one day at forenoon and said: Do you have anything (any food)?" She said: "No." The Prophet ﷺ said: "Then I am fasting. (Related by Muslim in his Sahih [Book of Authentic Hadith].)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.