"They have a daughter."
Translation:उनकी एक बेटी है।
I think if you look at what zeebo7 said, daughter is the subject here, so you use है, because it's singular, not plural. (But I'm no expert)
पास is never used with people, only for objects. The correct way to formulate it in Hindi literally translates as "Their daughter is", but is understood to mean "They have a daughter". Just a quirk of the language that must be memorized.
I am having problems with noun and pronoun agreement. Why is it उनकी and not उनके? Because it refers to a daughter?
I think the way it was written in the possessives lesson, that possessive pronouns should be matched to the object, so बेटी is feminine and so is the possessive pronoun उनकी !