One of my favorite tv shows in the 1960s was the sci-fi series "Lost in Space". The show starred June Lockhart and Guy Williams, and among the cast playing their children were Angela Cartwright, Marta Kristen, and Billy Mumy.
Billy Mumy played the role of the youngest child, 'Will Robinson', who was seen usually in the company of the nefarious 'Dr. Zachary Smith' (played by Jonathan Harris) and the 'Robot' as they explored the environs of the uncharted planet on which they'd landed.
A few years earlier, Mr. Mumy came to prominence on an episode of "The Twilight Zone" ("It's a Good Life") in which he played an all-powerful evil child.
In subsequent years, in addition to acting, he has been a musician, instrumentalist, and voice-over artist.
I sent this photo to Mr. Mumy in 1980, to his Los Angeles residence, and he returned it within two weeks.
Billy Mumy has his own website, and it can be viewed here for lots more info about his career and what he's doing today.