I'm wondering what Harpo & Susan's younger children - Alexander, James, & Minnie - have done in their lives. In "Harpo Speaks" he mentions they were interested in, respectively, rocketry, mechanics, and horses (of course, at that time they were teenagers). Did they indeed follow those early interests? Did they all stay in Southern California? I'm assuming they have kids of their own, probably grandkids now, too.
Susan (Mini) Marx lives in Orange, Ca. and has most of Harpo's old props. Billy Marx (the oldest) is an entertainer in his own right and really the only one that went into music. For their whereabouts now-a-days, contact Mel Zerman @ LimeLight Publications in NYC. It's a theatrical publishing house and the publisher of HARPO SPEAKS. He sends the "kids" their royalty checks whenever a new edition of that book is released.