Could "They had not known of their mother" also be correct? Or is it a different meaning/translation?
I'm not a native speaker, so take this for what it's worth:
I think you would have to use the verb "saber": Ellos no habían sabido nada de su madre.
Here's a link that explains the difference between "conocer" (to know, to be acquainted with, to meet) and "saber" (to know, to know how, to know about)
http://spanish.about.com/cs/vocabulary/a/verbs_for_know.htm "Conozco a Pedro." vs "No sé nada de Pedro." ("I know Pedro" vs "I don't have any news about Pedro.")
Why not "sus madre" to agree with the plural subject, if Duo was looking for "their mother"?
The preposition must agree with the prepositional object. The subject is irrelevant in terms of number of the preposition.
Technically it's correct, but it would be more likely to say "la madre de el" instead of "su madre". Generally when people use "su" they seem to be referring to the person you're already talking about, in this case, them.
I used "of" their mother, which was a prompt provided by duolingo. Any idea why this wasn't acceptable?
DL gives "known of" as a definition of conocido then marks it wrong in my answer.