"Where is the mother of those two children?"
Translation:Kde je matka těch dvou dětí?
This is the basic use of the genitive case which works in many other IndoEuropean languages.
This is second noun case used to express that the mother sort of owns the children. You don't use any preposition there.
Please use the name genitive and not the case numbers when helping (which is welcomed).
oh okay, thanks so much to both of you! yeah saying genitive helps me more because i think i learned accusative after nominative so i forget the order. :/