"These are children."
Translation:Αυτά είναι παιδιά.
Αυτά is used for people in Greek, it only has to do with the grammatical gender of the word children-παιδιά and it is really not offensive in any way. Αυτοί είναι παιδιά could be correct in the right context, but since extensive context is often missing in Duolingo I suggest sticking with the obvious rules for matching genders. :)