Here Oliver Cowdery initially wrote the singular cheek in 𝓟; but then virtually immediately he added the plural s by inserting it inline (there is no change in the level of ink flow). 𝓞 is not extant here but most likely read in the plural. Despite the correction in 𝓟, the 1830 compositor set the singular cheek. And all the subsequent editions have followed the 1830 reading.
As discussed under Mosiah 12:2, the Book of Mormon text favors the plural cheeks, as in Alma 14:15: “and the judge smote them again upon their cheeks”. Here in Helaman 4:12, the 1830 compositor may have been influenced by the King James language, which permits only the singular cheek in references to smiting someone. And most of these biblical examples are of the form “the cheek” while the Book of Mormon examples are mostly of the form “ cheeks”. For each case of cheek(s), the critical text will follow the earliest extant reading, thus here in Helaman 4:12 the virtually immediate correction in 𝓟, cheeks. In addition, the determiner the will be maintained in this instance even though their cheek(s) is more common than the cheek(s) in the Book of Mormon text. See under Mosiah 12:2 for a listing of all examples from both the Book of Mormon and the King James Bible that refer to smiting someone on the cheek(s).
Summary: Restore in Helaman 4:12 the corrected reading cheeks in 𝓟: “and smiting their humble brethren upon the cheeks”.