@silverwizard
They need to be 14 because that's their age! (I wasn't talking about level)
Good point. Still, could be a reason to let them play at level 14.

D&D's core Combat Thing is the Action Economy.
That's true. Well, if I understand correctly, a dragon has a couple of attacks: I think they can claw/claw/bite in melee, or do a big breath weapon attack, possibly on multiple targets. It also gets the free tail slap as a legendary action.

But my goal here is not to have everybody hack away at the dragon while the dragon just sits there and attacks them back; I want something more dynamic than that. I want at the start, to have most of their attacks be useless against the dragon, until they figure out how to hurt it.

Of course that also means they need to survive until they figure that out; they may need plenty of access to healing. I'm also seeing this as a fight with multiple rounds, where either they or the dragon breaks off and regroups to figure out a better way to fight.

And - yeah - listening in is hard - but I recommend trying.
Well, my son, who already knows about my dragon idea, seems to want to climb on top of the dragon. First he was planning to do it as a monk and then grapple it to the ground, now he's thinking teleporting to the back of the dragon as a wizard.

So having someone on the dragon's back is certainly something I'm going to take into account.