How to translate “这帮丫头片子，也不知道都便宜哪帮孙子了”?
'chicks' is a kind of unflattering nickname for 'girls', not a very accurate translation of '丫头片子' - '(immature, innocent, naive) young woman/ girl'.  However, in this context, the main character does sexualize them, Thus, both 'chicks' and 'young women/ girls' are a suitable translation
Personally, I would translate as follow:

这帮丫头片子 - These girls,/ This flock of chicks,


也不知道都便宜哪帮孙子了”. Who knows what bad boys are getting into their panties /  Who knows what bad boys are getting lucky with them

孙子 (grandkid) here is a 謔稱 (unflattering nickname) for men who are not mature or refine enough to be called upstanding
孙子 is seemingly a short form of 龟孙子 (unworthy rascals)-- a slang in Mandarin