Why ont dessiné instead if ont dessinés ...children is plural
No, because when the auxiliary verb is avoir, the past participle (dessiné) does not agree with the subject (les enfants) but with the object (les montagnes), and only if that object comes before the verb in the sentence, which is not the case here.
Nicely explained Jojo. Now to remember!
So now "mountains" are plural unlike elsewhere in these exercises where "mountains" were translated in the French singular!