"They are not children."

Translation: Ei nu sunt copii.

[BlackBouche]

¿Ele or Ei?

[LucasBauer19]

Ele is feminin, so if it was a group of all woman it would be Ele, but If it's a group with at least one man it's Ei, this doesn't really tell you so technically it could be either.

[IoanaPopa7]

Ei

[EmkQk5]

Why is it sunt in this case?

[Ces.10]

Because that is the form you use with this verb.

Eu sunt
Tu ești
El/ea este (e)
Noi suntem
Voi sunteți
Ei/ele sunt

Ei/ele usually shares the verb form with either el/ea or eu.