One Silmarillion storyline with a generally "happy" ending, and with battle
scenes (an important consideration if PJ does it
) would be a fleshed out
tale of Tuor and Idril.
But there are two others that would make very good childrens movies, with
lots of editing and picking and choosing from the books:
1) Roverandom
2) The Father Christmas Letters. The general theme here could be rather
like The Princess Bride, with JRRT writing and/or reading some letters with
his children finding and reading others (like Peter Faulk's grandfather
device in The Princess Bride).