"The grandchildren sang constantly. They kept singing!"

Translation: La genepoj ade kantis. Ili kantadis!

[hulten]

I think that "nepoj" should be accepted. Nowadays, the "ge"-prefix is typically only used when it is essential for the conversation.

[MartinShadock]

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.

[deactivated user]

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