Does the fasting of one who does not pay Zakat al-Fitr become invalid, and what are the rulings and regulations for paying it, and can children pay Zakat al-Fitr on behalf of their father who did not pay it previously due to ignorance and low income?

Zakat al-Fitr is an independent obligation that a person must pay on behalf of themselves and those they support, even if they did not fast Ramadan. It is a purification for the fasting person and a sustenance for the needy. Whoever pays it before the prayer, it is an accepted zakat; and whoever pays it after the prayer, it is a charity. If a person does not pay it at its due time despite being able to, they are sinful, and must pay it immediately on behalf of themselves and those they support. If they were ignorant of its obligation, there is no sin upon them, but they must make it up. As for its amount, it is a Prophetic Sa' (four mudds), estimated by the Permanent Committee for Issuing Fatwas to be approximately three kilograms. It is permissible to pay it in cash if there is a benefit in doing so.