Questions & Answers about Ellos son niños tristes.
Why is it tristes instead of tristos?
Why does the word tristes come after niños?
Since sadness is a feeling, why does this sentence use son instead of están?
You are right that the verb estar is used for temporary feelings, like ellos están tristes (they are feeling sad right now). However, this sentence uses the verb ser (son) to describe what kind of children they are. Saying son niños tristes means they are gloomy or melancholy children by nature as a personality trait.
Why isn't there a word for some (unos) before niños?