cases. kobieta is singular nominative. Mostly subject of the sentence and after "to" - Kasia to kobieta (kasia is a woman) Kobiety is plural nominative - the same

Kobietami is plural instrumental, at this point of course it follows "być" Jesteśmy konietami- we are women, one są kobietami - they are women,

You may want to read about instrumental case and "to" in the tips and notes defining https://www.duolingo.com/skill/pl/Defining

The word "dziecko" is neuter, so even if the particular children you're talking about are boys, the plural version is still 'not masculine-plural'.

And even if it was masculine plural, it would be "duzi". The masculine personal version of the adjective is usually quite different from the other ones.

'żi' does not appear in Polish in words from Slavic origin.

No, duże is plural big for all neuter, all feminine and masculine nouns that do not describe a person. We call all those not masculine personal, while men are masculine personal noun.

There are mainly two types of plural forms for an adjective. Either personal masc, and thr rest. In the case of the adjective 'duży', the personal masc plural form is 'duzi', and the rest plural form is 'duże'.

You have two plurals in Polish. One is "masculine personal" and the other is "not masculine-personal". The names say it all: first one is used for 'groups of people including at least one man', the other for anything else. The masculine personal form is "duzi", and the not masculine-personal is "duże".

"duzi" is the 'masculine personal' plural. "duże" is 'not masculine-personal' plural.

So "women" are definitely not masculine. No problem here. "men" are definitely masculine. Also clear. "dzieci" are more problematic. But the singular word "dziecko" is neuter, so this decides on "dzieci" being 'not masculine-personal'. Even if the children you're talking about are only boys, if you decided to say 'children' and not 'boys', the right form is "duże".