That being said, my kids liked it. But my 5 year old liked it more than the 9 year old because the older one was a bit bored by it.
I think you're being too critical, especially if you are trying to compare them to the LOTR trilogy.
These book are on average less than 200 pages each. How involved did you expect them to be? And how could you expect that they would contain the same level of detail as the LOTR books, which are all at least 4-600 pages long?
At least you did read them, and your opinion is based on some actual knowledge of the stories. And since you didn't much care for the stories, I can't really expect you to enjoy the movie. I'm glad your kids liked it.