"Little boys do not like soup."
Translation:Mali chłopcy nie lubią zupy.
You use the genitive case for the object of negated verbs only if the non-negated verb would use the accusative.
So if the non-negative verb puts the object or "complement" into any other case apart from accusative (e.g. locative, dative) then you continue to use that case.
Why is it mali and not małych? I remember reading in the Tips that adjectives and determiners usually use -ego/-ej/-ych, and mały was given as an example.
Is it because "mały" is not the word being negated - lubią is? Although I don't understand that entirely, because I thought the entire sentence had to be in the same case (unless separated by a comma)?
Zupy is the negated direct object, that's why it takes genitive. Mali chłopcy is the subject, so it's nominative virile:
Gender – „chłopcy” is masculine-personal so connects with „mali”, „chłopaki” is a broken word that I strongly suggest to never use in plural… But for the sake of completeness, it is not-masculine-personal in Nominative and acts like masculine-personal in other cases. Since 'boys' are the subject of the sentence here, so they are in nominative case and therefore „chłopaki” connects with „małe”, which is the not-masculine-personal inflection of the adjective.
Well, then what would you use to say "boys" in plural when they're too old to say "chłopcy"? Of course this word behaves a bit strangely, but it's not such a difficult exception to grasp...
Anyway, "małe chłopaki" sounds weird to me, as if they were literally 'small', miniaturized.
What I would use is immaterial, as I am native, so I don't have any troubles with it – I'm just saying, that nothing terribly bad will happen if a Polish learner will use „chłopcy” in such a situation and while it might not be hard to grasp, remember that there are literary hundreds of other things(including many exceptions) that a learner has to burden his memory with in order to use Polish.
I just think that it would be easier from non-native perspective to not use this word, at least early in their experience with Polish. ;)