Praise be to Allaah.
What you must do is stop him, because of the general meaning
of the shar’i evidence concerning that, including the words of the Prophet
(peace and blessings of Allaah be upon him): “When one of you prays
facing something that is screening him from the people, and someone wants to
pass in front of him, let him push him back, and if he insists then let him
fight him, for he is no more than a devil.” Agreed upon. What is meant by
the phrase “let him fight him” is let him push him back forcefully. And
Allaah is the Source of strength. End quote.
Majmoo’ Fataawa Ibn Baaz
But if the child is passing frequently and stopping him will
distract the worshipper from his prayer, then he should let him be and not
focus on preventing him.
Al-Haafiz Ibn Hajar said in Fath al-Baari:
Ibn Battaal and others narrated that there was scholarly
consensus that it is not permissible for him to move away from his spot in
order to push him away, or to move a great deal in doing so, because that
has a greater effect on the prayer than his passing in front of him. End
And Allaah knows best.