Of course. Deathly Hallows' problem wasn't that it was simple. Indeed, you could actually say it was too complex! The problems involved are with poor storytelling, which would be the same problem in an "adult" or a "children's" book.
I did read though where Rowling intended the books to increase in complexity, so that the readers would "grow" with the characters. If nothing else, the increase in page count would bear that out!
Dead-on. I actually made it thru the entire series, altho I felt like I was whipping myself with a cat-o-nine-tails just to get thru it. Personally, I really felt like it was poor writing all the way thru, even if you want to give it the crutch of being a "children's book". Hell,
The Hobbit was originally a Children's book, and the quality of writing is such that it is entirely enjoyable by adults.
Another Children's series was Lloyd Alexander's
Prydain stories, written in the early 1960s. This was a very Tolkienish type tale, altho clearly written for a younger audience. However, the writing was still intelligent, and it dealt with real issues of danger, injury, and death, as well as obligations that come with growing up. Even as a children's book, I think most adults who enjoy fantasy reading would appreciate these stories. I re-read these when I was 30 years old, after being away from them for probably close to 20 years, and I felt they were still a good tale.
It's all about good writing, which, in my humble opinion, was severely lacking in the Potter books.
Where I think they went wrong in making the movies was that they didn't wait until all the books were published first. So as each volume was delayed, that delayed production of the movies, and now we've got 25-year-olds playing the part of 15-year-olds, and it's just painful. And those kids have changed in ways that aren't consistent with the books. Anyone notice how in the books thru to the end, Neville Longbottom is portrayed as being heavy and bumbling? Did ya notice in the last movie how the actor who portrays Neville has grown up and he's tall and thin?
They rushed this into movie production before the story itself was complete, and that sort of obligated them to make ALL the books into movies or it would be even more incomplete. And the result is a weird feeling that it was all forced along, in some unexplainable sort of way.
If they had waited until all the books were published, they could have had a more coherent master plan, sort of like Jackson did in making LOTRs.
Anyway, I've been none too impressed with either the books, or the movies.