"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.
As you can read in the Tips and Notes, Polish doesn't have articles (a, an, the).