"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.

[vikungen]

Not with family words, as they are gendered.

[hulten]

I see, I'll remember that.

How about other words? I am used to say "amikoj" instead of "geamikoj", even if they include women. Is that accepted?

[vikungen]

Yes, most will agree that amiko is not gendered.

[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!