When I wrote, "How many sullen children are at the playground," I was caught for using the wrong word, but only the "e" of "are" was underlined. Similar things often happen. I hope they will be corrected in beta.
My answer: How many sulking children are on the playground? This should be accepted. They could have been, sullen or peeved, too. In fact, nuha is heard more often to describe a sulking person. Please adjust this Duolingo and don't get nuha about it.