"The child has small black pants."
Translation:L'enfant a un petit pantalon noir.
You cannot tell by the English that it is one pair of black pants. "L'enfant a de petits pantalons noirs." should also be correct.
The issue here is not whether it is singular or plural. The plural version should be "L'enfant a de petits pantalons noirs." because "des" becomes "de" when there is an adjective before the noun. I had forgotten this and put des and got it wrong too . . . sigh.
There should no 'un' in the translation. Otherwise it should read 'The child has a small black pants
No, you cannot expect to translate word for word from one language to another. In English we could say “The child has a pair of small black pants.”, because the French “un pantalon” equals “a pair of pants”, but we often just say “pants” even for one pair of pants or trousers for the British. We cannot say “a pants” in English.