I sort of see that as mediocre writing, not necessarily 'bad'. I see bad as mistakes, words/passages that don't add anything, overly descriptive/lack of description, if it reads like a thesaurus was used for every word (not because I don't understand big words, but sometimes it seems unnecessary or forced)...
I think when writing for children, it's difficult not to get into a he said, she said pattern, and it is sometimes necessary to add description to how something was said because a child might not understand the context even from the reaction to what was said, especially in a fast-paced children's book when attitudes and emotions change quickly. If, as in the article's example, Harry goes from furious to glum in less than a page, then a child might miss that.
I don't think that makes the books bad, but I guess if I read a book in that writing style when I wasn't aware that it was supposed to be a children's book, I'd see it as poorly written. I find chick-lit a bit like that sometimes.
Basically, I can't imagine great writing in a child's novel. I haven't read any Roald Dahl as an adult. Hmmm... I've got The Hobbit upstairs, so hopefully that'll be good.