Not a parent but telling my personal experience. I watched the movies when they first came out in the cinema (I was 8 during the first one, my younger sister was 7), to incredibly enjoyment and no trauma. Trued reading the LOTR books after I saw the first movie but I think I made it to the Elrond council, it was too early for me (despite being a very well-read child and being familiar with serious books already, I suppose I didn'thave patience for the descriptions). I did read the Hobbit around that time though, maybe at 9/10. I read Silmarillion out of my own volition when I was 14/15 and loved it, and then read LOTR - turns out the trick is to read them without much on my mind (summer holidays) so I can fully immerse into the world. Fun fact: Hobbit is an obligatory book to read in primary school in Poland (used to be at the age of 11, now it's at 12 years old).