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

                From what age do we have to prevent a baby/small child from passing between us and our sutrah,further if despite our efforts to stop them they still pass has our salaah been broken?.
Praise be to Allah.
    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.