Is what this worshiper did permissible, by hitting the children and expelling them from the mosque, and telling them, "Your prayer is not accepted"?

The prayer of a discerning child is valid if it fulfills its conditions and pillars. There are benefits to bringing children to the mosques. Children are commanded to pray, as a recommendation, from the age of seven and are to be disciplined for it at ten, so that they become accustomed to it. Children should be contained, taught, and made to understand gently and kindly if they make mistakes, so that they love the mosques, because gentleness adorns everything. A man hitting your children constitutes grave violations, among them: violating the sanctity of the mosque, punishing those who do not deserve it, alienating people from mosques, humiliating the face which the Prophet forbade striking, and speaking about Allah without knowledge. The man should be advised to repent and his violations should be explained to him. If you fear corruption, seek assistance from someone you trust, and continue to bring your children to the mosques while teaching them their etiquette.