Norman D, thanks for correcting my wee-hours-of-the-morning mis-post.   

Yes, it's Rufus (Wainwright) who is Kate's (and Loudon III's) son.   I couldn't think of his first name at the moment, so I googled "Kate McGarrigle" and "Loudon Wainwright" and found a blurb that gave me this: "Kate is the McGarrigle who, with Loudon Wainwright II, spawned the wonderful Loudon Wainwright III, too."

This morning I realized my mistake and was about to post the correction.   You beat me to it. Thanks.

Genie