I am wondering if you can't just drop the first 'boys' in English because it would sound better, but I understand if it is just to make sure it is translated almost literally
I would say that it depends on what a person saying the sentence want to emphasize.
You could say "The big and little boys" (Duzi i mali chłopcy) when describing a group of boys of random height.
At the same time you could say "The big boys and (the) little boys" (Duzi chłopcy i mali chłopcy) when you want to distinguish two groups of boys depending on their height.
It all depends on the context.
Is it not dużi chłopcy? according to this ż in duże changes to z in plural, I am seeing this change for the first time, can someone confirm?
As far as grammar is concerned, the only thing that changes, is the ending, which becomes -i. The rest is just phonology and spelling conventions. In Polish, "i" is an palatalizing (softening) vowel, which means that it softens at least one preceding consonant. So in this case, ż is being softened to ź. Furthermore, spelling convention dictates that ź,ć and ś are written without the diacritical mark, when followed by the letter i. This why it's duzi, but the z is still pronounced as ź.
Here you'll find some more rules in the comments.