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

Question

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.