"Lambs are offspring of sheep."
Translation:Ŝafidoj estas idoj de ŝafoj.
The rule I saw elsewhere is "The main difference between the use of the definite article in Esperanto and in English is that in Esperanto the article, with a singular noun, may be used to indicate an entire class." I picked up that habit from Bulgarian, and it seems it is standard in Esperanto as well. In this case the direct translation from English (without article) strikes me as less correct. But the rule does say "may," not "must."
The reason the article to designate an entire class is helpful is because Esperanto lacks the indirect article ("a"), so if you leave off the article when talking about a class, it's ambiguous whether you are referring to the class or an individual member of it.
Ugh, that got technical. But it is what I was thinking!
"Offspring" does not have a plural form. It's like "sheep" that way.