I’m wondering whether they had some sort of genetic anomaly like marfan syndrome. I remember that Jim Hendrix was tall. Was John the baby in The Labyrinth? I think he or his brother (if he has one) was the baby. We loved that movie. “You have no power over me.” sad that he died.
As a child, Henson appeared in his father's Numerosity films for Sesame Street as a boy showcasing "10 bells" and in the film "Memories" for Bufferin as the narrator's son.
http://www.youtube.com/watch?v=ts1WvLWbwEw