"Ces grands-parents ont douze petits-enfants."
Translation:These grandparents have twelve grandchildren.
"Kid" (un gamin/une gamine) is not accepted for "enfant" (child) because of the register of speech.
As a consequence, and for consistency, Duolingo cannot accept "grandkids" for "petits-enfants".
Note that "grandkids" does not have a direct translation in French.
You can't, they are homophones. To avoid this issue, I disabled the "type what you hear" exercise.
Twelve grandchildren ?....Poor grandparents ! I have four delicious grandchildren and it is enough for me !