When should a child be stopped from passing in front of a person who is praying?

The Muslim should strive to have a sutrah (screen) in front of him when he prays, whether he is leading the prayers or praying alone. He should also try not to let anyone or anything pass in front of him, because the Prophet (peace and blessings of Allaah be upon him) said: “When any one of you prays facing an object that he is using to screen him from the people, and someone wants to pass in front of him, let him push him away, and if he insists let him fight him, for he is no more than a devil.” (Narrated by al-Bukhaari, al-Salaah, 509; Muslim, al-Salaah, 505). 

With regard to young children, they should also be prevented from passing, because of the general meaning of the evidence and because it was narrated from ‘Amr ibn Shu’ayb from his father that his grandfather said: “We stopped with the Messenger of Allaah (peace and blessings of Allaah be upon him) in Thaniyat Adhaakhir and the time for prayer came, so we prayed facing a wall. The Prophet (peace and blessings of Allaah be upon him) faced the wall and we were behind him. An animal came and wanted to pass in front of him, and he kept pushing it back until his stomach touched the wall, then it passed behind him.” (Narrated by Abu Dawood, 807; classed as saheeh by al-Albaani). This indicates that stopping others from passing in front is not restricted only to stopping adults or those of sound mind. 

But if the worshipper tries his hardest to stop the one who wants to pass in front of him and is unable to stop him, what is the ruling in this case? 

Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

“The sin is on the one who passes in front of him. If you did what the Prophet (peace and blessings of Allaah be upon him) commanded but were unable to stop the one who wanted to pass in front of you, then your prayer is not affected.” 

Al-Sharh al-Mumti’, p. 343.