Hi, and welcome!
My 6 yr old adores LOTR. He has them almost completely memorized, as well as the Harry Potter series. Watching HP right now on network TV. Course, I don't know *why* since we have it on DVD.
LOTR, and HP, are both mandatory watches at my house.
Although, ( and I may get burned for this) I have yet to see ROTK.... I've got the DVD, but have no player. I was going to preview it before letting 7yo watch it - the Pellenor Fields, I'm told, was even bigger than Helm's Deep...