Why is dårlig not plural (darlige)?
"Dårlig" is functioning as an adverb here, not an adjective, so it does not decline.
Du er snill Deliciæ
Is dårlig not intense enough to mean terrible/terribly?
"Terrible/terribly" would be "fryktelig" or "forferdelig", which indeed have a stronger meaning.
Connor, the app won't let me reply directly, but I think it's because you're describing the behaviour not the children.
I think it's because it's an adverb (so yes, describing the verb – behaving) and not an adjective (describing children).
I heard/wrote "barnet" (singular). Why is that wrong?
For listening exercises, only the sentence being read out will be accepted, which in this case uses "barna". I agree that the audio is a little ambiguous on this one though.
Here could I use the phrasal verb 'act up' ?
Yes, that's a good fit.
Children behaving badly? NEVER!!!!