It was a children's book and probably isn't anymore.
Rest assured, I can personally confirm that it is still a wonderful children's book.
I read it to my daughter when she was six, and she loved it. We did one chapter a night. I did almost no editing as I read.
By the time we read LOTR she was eight, and we never did finish ROTK because the Frodo and Sam parts really do drag on (I get that he wrote them this way so that the reader would get a sense of just how arduous the journey was, but...)
How could that status ever chance? Being widely read by adults doesn't change if its an children's book or not.
Was its license rescinded by the International Society of Children's Books? Thanks for letting me know, I'll be sure to tell my child to stop enjoying it.