"des" is the plural of "une": une baguette - des baguettes (countable)
"de+les" = "des" is used with verbs constructed with preposition "de" : je parle des baguettes (parler de)
"du, de la or de l' " are partitive articles used with singular, mass nouns: je mange du fromage, de la viande, je bois de l'eau (uncountable).
Duo is becoming lenient, as it seems, because the French sentence proposed for translation should, on principle, be translated according to the same register of speech:
formal to formal: "mangent-elles/ils des baguettes ?" or "est-ce qu'elles/ils mangent des baguettes ?" = are they eating baguettes?
relaxed to relaxed: "elles/ils mangent des baguettes ?" they are eating baguettes?
Est-ce que. When you have est-ce que, you don't really invert the position of verb and subject. "Est-ce qu'elles mangent des baguettes?" But when you don't use "Est-ce que", it's the grammar rule to invert like this one above (Mangent-elles des baguettes?).
Although, I heard that, in spoken French, people just ignore inversion and ask questions without inversion (declarative way) sometimes, using only intonation to tell it's a question. Well it's complicate, but we just have to agree with the rule. :)