I just finished reading the LotR series with my now-11 yo son (we started a few years ago) and there were definitely plenty of sections like this that I converted to more understandable terms and concepts on the fly. Sure my son may not have heard this exact line, but he definitely got the idea that Shelob was old, bad, and Sauron had encouraged get 8 in exchange for a scary guard in the tunnels into Mordor the fell beasts were scary.

Edit: apparently my son may or may not have gotten an entirely different version of the book told to him!