Why not "...leurs joues" ?
There are specific rules for body parts. Basically, as soon as you know who the body-part owner is, you change the possessive to a definite article.
Could children have chocolate on someone else's cheeks?
An exact transcription of the sentence would, indeed, be "leurs joues" ; but an embellished sentence, to make sound it french and common, well "sur les joues" would be used. But both are equally correct.
A tautological sentence is not equally correct when compared with one which is not.