The Prayer of Children in the Mosque

Question :

Is it permissible for children of less than ten years of age to pray in the mosque in the middle of the row, when they are wearing short trousers or diapers worn by those of three or four years of age, and is it permissible for the Imam to pray in a sheet?

Answer:

It is permissible for a boy who is rational and reasoning to pray in the row after he has been trained and he has learned the deference and respect required by the mosque and the worshippers, with the condition that he does not indulge in horseplay, and that he is in a state of complete physical purity. It is better for the boys to be behind the men, unless it is feared that gathering them together will cause a lot of horseplay and laughing, which will annoy the other worshippers, then they must be separated. As for those under the age of reason and rationality, they should not be allowed to enter the mosque at prayer time or during the Khutbah, because they do not understand the sanctity of the mosque. And regarding the prayer of the Imam or anyone else in one garment, such as a Rida', that is a sheet, there is no objection to it, as long as it covers the 'Awrah; but it is better to wear an Izar or trousers under it, to prevent any of the 'Awrah from being revealed.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 220-221