A dozen = une douzaine.

"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.

A dozen = une douzaine.

You can't, they are homophones. To avoid this issue, I disabled the "type what you hear" exercise.

Probably not. In Cheaper by the Dozen it was children, not grandchildren.

I have but one. Sob. Although he is delightful and lives next door.