probably a dumb question
How do you know when they are talking about girls/kids or similarly boys/kids
Girls/kids ends in 'as'. Boys/kids ends in an 'os'. If you are talking about a mixture of boys and girls then the 'os' ending is used.
Usually you would know this from the context of the bigger text the sentence would appear in. It seems duolingo treats them as synonyms in the exercises.
Sarahpw is spot on when translating from Spanish.
If you're translating from English, then kids could be either. I would suggest making niños your default, since it's more often correct, and I think would be used for an unknown case (Tienes niños?)
When the word is singular, -o means boy and -a means girl. But when the words are plural, -os means all males or a mixture, and -as is reserved for exclusively female groups. As in English, the masculine form is the default or general form (with the exception being éste/ésta/ésto. éste is masculine and ésto is neutral)