"Which child is it?"
Translation:Které dítě to je?
Sometimes Duolingo offers "correct" answers that seem strange. This usually happens when we type something that is not quite right, but is similar to another acceptable translation. Your answer, "Čí je to dítě," means, "Whose child is it," which is why it was not accepted. But since it's structurally close to "Jaké je to dítě," which is accepted, Duo chose that as the correction to show you.
Both "Které dítě to je?" and "Které je to dítě?" are accepted as the correct translaton of "Which child is it?" but I would not use the latter to express something like: "Which child among this group of children is the one you have mentioned before?".
On the other hand I would use "Které je to dítě? (with or without a stressed "to") to say "Which one is that child?" (e.g. on the list).
There is a problem with the previous listening exercise Are Matěj and Kateřina husband and wife, requires input in Czech but refuses to accept Czech and reports that you have written in English! No facility to report problem on that page and only way to carry on is to turn off listening!
I answered that several times, but it is hard to find. Often you can use either. When pointing to/introducing some new object you use "To je ..." (To je moje matka. To je ten dům?). When speaking about something already discussed ("It is big." "It is a large house.") you will more likely use "Je to ...".
That only concerns if you use it at the start of the sentence. For example, "Které X to je?" and "Které X je to?" are pretty much equivalent.