Translation:Children do not drink beer!
Well, they shouldn't at least
why can't this mean 'the children do not drink beer' ?
Oops, that's an oversight. Added versions with "the".
What makes the ending of this sentence piwa instead of piwo?
"pić" (to drink) takes Accusative, here it's negated, so it needs Genitive. And Genitive is "piwa".