I don't understand, is this an exception? usually 'the + plural' is simple an ene ending on the noun. why here do we need to add ''de"?
It's because "voksne" isn't a noun, it's an adjective moonlighting as one. Adding "den" (sl.) and "de" (pl.) to the adjective is one way of turning it into a noun. You can think of it as having an implied "people" following it:
"de voksne (menneskene)"
Our default translation uses "cry". If you want a definite response in cases like this, you need to copy-paste the entire answer; the issue may lie elsewhere, and often we're blind to our own typos.