I always hear un noise at the end of the word but why is not it waladun instead of walad
The "-un" at the end of words is used to indicate that the words are in the nominative/subjective case. "Nunation" is used in MSA but not in Arabic dialects. Sometimes Duolingo uses nunation, other times it doesn't.
Yes that looks like a mistake to me too. Unless we're missing something?