"Les enfants mangent des pâtes."
Translation:The children are eating pasta.
A simple heuristic is to think of "de" (and other d- articles such as "des") as meaning "some". It stands for neither "one/some" (un/une/...) nor "the" (le/la/...). Since French seems to always need an article, when you have to choose something other than the definite and indefinite, you pick the de-family. (At least, that's what I've been able to infer so far.)