Translation:The child asks: "Where is mum?"
It rejected the child asks where is mom?
Apparently it's rejecting lack of a colon, quotation marks, or a capital W. I have never noticed it rejecting anything for punctuation or capitalization. I have just started using Duolingo a lot again after using it very little for a good while. Have they started caring more about punctuation or capitalization?
"Mum" is British. "Mom" is American. They're otherwise entirely synonymous. Either the dev team only coded in one but not the other, or Chuck hit a glitch.