I submitted 'there are children at the garden', and I'm rather baffled at the 'advice' error message, saying 'In English, you are normally "in" a garden, not "at" a garden.' I would disagree here. The use of the proposition in vs at in this case is both correct in my opinion, with preferences to euphony or context. For example:
The child arrived at the garden, not The child arrived in the garden
For proper nouns: The child was at Covent Garden, not The child was in Covent Garden.
This seems like a rather arbitrary rule that Duo has set, especially galling given the flexibility and arbitrariness of prepositions (not just in English).
I incorrectly wrote "the children are in the garden". How would that be translated?
I'm not saying it shouldn't be accepted; it probably should.
But it's actually trying to teach you a point: Placing the locative first in this Turkish sentence is the most neutral and unmarked way to do it. The most neutral and unmarked way to do it in English is "There are children in the garden." Moving that prepositional phrase up front shifts the focus of the sentence. If you try to translate everything piece by piece without moving them to the language appropriate slots, you'll miss out on the fact these rules exist.
Comically enough, I would say a better way to render "in the garden, there are children" would be "çocuklar var bahçede" though a native might disagree. :)