This is the 1959th most frequent Polish word.
chłopców
"Chłopców" means "boys" (genitive or accusative plural form) in English.
The word 'chłopców' is used here in the genitive plural form as it complements the direct object of the sentence under observation.
Widziałem grupę chłopców bawiących się w parku.
I saw a group of boys playing in the park.
The term 'chłopców' is in its genitive plural form as it comes with the negated verb 'poznać', requiring this case.
Nie zdążyłem poznać tych chłopców na przyjęciu.
I didn't get the chance to meet those boys at the party.
In this case, 'chłopców' functions in the genitive plural due to its role in indicating the subject of the mother's concern.