"The woman and the man have a child."
Translation:Kobieta i mężczyzna mają dziecko.
Well, usually first the man and the woman have sexual relations, although there's of course in vitro or adoption as well...
But seriously - "dziecko" is simply the Polish translation of "child". OK, it's in Accusative here, but neuter nouns have Nominative and Accusative forms identical, so nothing changes, there isn't anything to explain.
I think this table should answer your question: