the real unpopular opinion here is 'why can't they try to find something to interest them that's within the curation rather than needing to be something from outside it?'
Well, in the given example of a small child... you're not really going to get anything out of playing D&D with a small child except
playing D&D with a small child and your carefully curated setting and your painstakingly assembled themes are going to get chewed up and spat out. There's only two kinds of D&D you can play with a small child: the unfettered whimsy of someone who doesn't understand the mythic significance of Tolkien's cosmology, or the unfettered savagery of a pre-moral intelligence trying to prove how very grown-up they are. Real talk, you're going to get a lot of both and not much else.
I highly recommend it, it's a lot of fun, but who are we really kidding here about the artistic integrity of that game?
On the other hand, some people only want what they can't have and the only reason they want
is that they can't have it. When the "small child" is 35 and agreed to play
Dark Sun but now insists on playing a Gnome Paladin, they need to be firmly but cordially invited to play at the kids' table until they're willing to play the same game everyone else agreed to.
I mean... yeah.
A lot of home referees are
way too precious about their generic "custom" D&D settings that are just a portion of the core D&D rules draped awkwardly around the carcasses of their favorite three book series. And they maybe need to get a grip.
But if you want an unpopular opinion for this thread? Limitations and restrictions are the basis of
flavor when comparing similar but not quite identical D&D settings. If you strip them away to accommodate the player who just
can't imagine playing anything but Favorite Character XXXVII, you might as well just throw the rest of the book in the trash. And that player is
not entitled to unilaterally force the group to play a different game than the one they agreed to.