"We are looking for the families of those people."

Translation: Hledáme rodiny tamhletěch lidí.

[Daniel-U]

Should this be "Hledáme rodiny tamhletěch lidů"?

Lid is masculine, and all masculine nouns end in "ů" in the genitive.

[VladaFu]

In plural it follows the feminine declension (like kost).