Well, here’s a surprise. I figured since the kid insisted we watch Space: 1999 from time to time, I’d do up its most infamous child-scaring episode right, and we watched it together late at night with all the lights out. “Dragon’s Domain” is the one with the great screaming tentacled monster with the headlamp eye that skeletonizes its victims. But our boy is a much older boy than the boy who was once so very bothered by many of the monsters we’ve seen together. He was flatly and firmly unimpressed. So nine’s too old. You got kids of your own? Throw this at ’em earlier.
The kid said that he liked precisely two things about it. Recovering from getting clobbered the second time by the “Saint George” character who insists on fighting his dragon, Alan asks “What’s that guy got against me?” And among the models in the spaceship graveyard, our son spotted the same ship used by Julian Glover’s people in the previous episode, “Alpha Child.” That’s it.
But I thought this was the best of the first eight by a mile. I really like its scope. Much of it is a flashback to an incident in 1996-97 where “Saint George” takes off on a 14-month flight to visit a new planet in the solar system, along with Michael Sheard and two women. They find a graveyard of other spacecraft, but “Saint George” can’t get out of the cockpit while Sheard and the ladies are horrifically killed by the monster. The dude escapes, jettisoning the bulk of his ship, makes it home, and nobody believes him. 800 days into Moonbase Alpha’s journey, in between galaxies and nowhere near anything, “Saint George” has a nightmare of the monster again, because the big dude got hungry and parked his spiderweb of spaceships in the moon’s path. Seems a bit unlikely that a big dude powerful enough to do that could get whipped by an axe to the headlamp, but there you go.
Here’s the other thing I really liked, and it’s the show that Gerry Anderson and Lew Grade should have given us instead of this silly series. Douglas Wilmer plays the commissioner of Earth’s unified space program, and there’s a hell of a show here about putting together the funds to explore our own solar system, and finding seven or eight derelict alien spaceships on the other side of Pluto, with or without a big space monster. It’s somewhere that Anderson kind of looked at five years previously in his strange feature film Journey to the Far Side of the Sun, but abandoned in favor of weird post-Kubrick metaphysics and philosophy about the strangeness of space. There’s even a little cameo by Bob Sherman, who’d later play the CIA guy in The Sandbaggers, as a newsreader for a show about the space program. I just think it’s a huge missed opportunity, because honestly, the nuts and bolts of how Moonbase Alpha got started is far, far more interesting to me than black suns and space rocks and rules of Luton.
But maybe I wouldn’t be focused on that had the big monster scared the pants off our son like it was meant to.
Fire Breathing Dimetrodon Time 
I watched this episode for the first time when I was in my 20s and I thought it was scary as all heck. I recently rewatched it and it still holds up very well. It sounds like your son is getting jaded at much too young an age.
Fun fact: One of the female scientists on the spaceship was played by Barbara Kellerman, who later appeared in the final Quatermass serial (1979) and played the White Witch in the BBC’s adaptation of The Lion, The Witch and the Wardrobe (1988).