With regard to smacking a child for not praying, it is stipulated that the smacking should be light and should not be painful and should not break the skin, or break a tooth or bone. It should be on the back or the hand and the like, and the face is to be avoided because it is forbidden to strike it, because the Prophet (blessings and peace of Allaah be upon him) forbade that.
It should also not be done in front of anyone else, so as to protect the child’s self-esteem and his dignity in front of his friends and others.
Ref: https://islamqa.info/en/127233
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Sahih Muslim: by Imam Muslim. Volume: The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa’l-Adab)
6321: Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: When any one of you fights with his brother he should avoid striking at the face.
6324: Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: When any one of you fights with his brother, he should not slap at the face.
6325: This hadith has been transmitted on the authority of Abu Huraira and in the hadith transmitted on the authority of IbnHatim Allah’s Apostle (may peace be upon him) is reported to have said: When any one of you fights with his brother, he should avoid his face for Allah created Adam in His own image.
Ref: http://www.theonlyquran.com/hadith/Sahih-Muslim/?volume=32&chapter=30