"Is he your grandson?"
Translation:C'est votre petit-fils ?
I don't think Duo is alone in teaching this rule. At least two French experts online also teach this rule regarding the use of c'est vs. il est / elle est with a modified noun.
The most relevant section is part A.
The relevant section is the table comparing the use of il est and c'est with unmodified nouns and modified nouns.
Finally, a French language instruction book also puts forward this rule:
"When a noun is used with adjectives that modify or refine the meaning of the noun, c'est is the appropriate choice. [as opposed to il est] Even a single article used with a noun is enough to modify it and make it necessary to use the c'est construction."
Just a remark "est-ce.. ?"without the "que c'". Indeed, grammatically:
Est-ce <-> Est-ce que* *c', the ce (of the first one) becomes the c' (of the second one), not the ce (of the second one).
And indeed we used it a lot and even overused when we grammatically shouldn't use it.
You're missing the verb. Est-ce que is a construction --- that goes all together in one block, you can't split it --- to ask questions. You can't translate it with words into English, it's just used to form questions.
Is he your grandson? <-> Est-ce votre petit-fils ? = Est-ce que c'est votre petit-fils ?.
I bolded the (real) verb+subject of the sentences.
For me the literal meaning "is it that..." works to translate "est-ce que", in the sense that it tells me how the construction works in French. You are correct that it is not standard English, though. (Both "Is it true that he is your grandson?" and "Is it the case that he is your grandson?" are valid English, but the literal "is it that he is your grandson?" is not.)