Because then the phrase would be 'Somos hijos.......' instead of 'Somos los hijos...............' the 'los' tells you that we are 'THE' children of humanity, as if there are no others. If there was not the 'los' it would be an all round 'We're all the children of humanity'
The reason it was marked wrong is easy - it hasn't been entered into the list of correct answers. The hard part is deciding if it should be. I'd say yes. It still conveys the meaning even though to me it sounds a little odd in English. It is perfectly literal, but that's not usually a good thing to shoot for in real world translation.
Yes. Niños refers to their status as young people (chronological age). Hijos refers to their status as somebody's offspring. So, “Children are fun." would use 'niños' because it's just 'young people' (in general). “Your children are fun." would use 'hijos' because you're referring to their status as somebody's offspring.
Hijos are hijos forever, by the way. Sons and daughters don't cease to be offspring once they've matured. Niños is specifically about an age and development.
Yes, niños is "children" and hijos are like sons or a mixed group of sons and daughters or like saying MY kids, or YOUR kids. OR you could say "mis hijos son bonitos" MEANING my children (mixed group of sons and daughters) are handsome/pretty. Or" my sons are handsome". It wouldn't be correct to say:mis NIÑOS son bonitos.