I have never heard either "milk with cocoa" or "cocoa milk" (which the correction function gave me) used in English. We just say "cocoa" both for the powder and the beverage made from it.
It is a Romanian type to say that and Duo gives this way for correct understanding of Romanian grammar, I think.
The translation it gives below says kid, but when you select the word in the sentence above yours is says child....which is it?
Shouldn't "chocolate milk" (instead of milk with cocoa) also be accepted?