From the ABC Saturday Superstar Movie:
Bewitched had ended after 8 seasons, and reruns were airing 5-6 days a week on ABC. Though no longer united under the Columbia tree, Screen Gems & Hanna-Barbera collaborated on an animated spin-off pilot, Tabitha & Adam & the Clown Family, which premiered in December 1972.
The Clown Family, you see, are relatives of the now-teenage Tabitha & Adam Stephens. I'll try to explain the growth spurt shortly. Anyway, I believe this was meant to be a pilot for a series, but it just didn't pan out. As we all know, Tabitha & Adam would experience another growth spurt a few years later............
We've previously featured the Clown Family's "Love The World", but right now, scope the complete episode. The first 8 minutes or so are replayed after the closing credits. Don't ask why.
Edit: 12/20/17: The video has been deleted.
Gene Andrusco was also working on another 1972 frosh, The Amazing Chan & The Chan Clan, over at CBS, and it certainly sounds like Austin Roberts, using the faux-British accent he created in recording songs for Scooby-Doo two years earlier, was the uncredited vocalist, a couple of years before he'd finally hit the charts with "Rocky".
Anyway, let me try to explain the growth spurt of Tabitha and Adam. My theory is built around Marvel Comics' concept of time for some, if not all, of their characters. Bear in mind that Tabitha & Adam were not yet 10 years old when Bewitched ended a few months prior to Clown Family. Let's assume that from the point where Tabitha debuted, the series covered about one day in her life per episode, and the same would apply to Adam a couple of years later. To try to explain how they would add another 10+ years four years later in the initial Tabitha pilot is a little more difficult. The familiar voices heard in this show include Paul Winchell, John Stephenson, and Len Weinrib.