"The grandchildren sang constantly. They kept singing!"

Not with family words, as they are gendered.

In the variant of Esperanto named Iĉismo, it would indeed be correct. It is not yet widely accepted, though. I guess that Duolingo wants to stick to the traditional Esperanto, which is understandable.

Anyway, we are not told whether these grandchildren were all boys, all girls, or a mixture!