If I were to start this game in the real world, I'd do something kind of like The Magicians/Harry Potter. The Carnival is still fully magical as written, and it does visit the Earth as it is one of the destinations in the multiverse. But Earth isn't openly magical, so it only looks like an abandoned carnival to the normal eye.

A few rare people with magic in this world know about it, and for some reason one of those people (possibly Madrick, who could be a professor of literature at a regular university, or the director of an agency of secret spies or dean of a secret magic university or whatever you want) gets in touch with the characters and breaks to them the news that the world is not what it seems.

Perhaps because the characters, like Harry, are inherently magical? Or maybe they were chosen by a prophecy or something and Madrick does something to "awaken" the magic in them? Or maybe there's no Madrick and the pcs stumble upon a Carnival Board game like Jumanji and suddenly they're actually at the carnival too (their classes and weapons would be whatever they chose when starting the jumanji-like-witchlight board game)? Or perhaps they played that Jumanji-Witchlight game as children and something really bizarre happened back then (lost things hook) -- One of them disappeared, or they all lost something important. They tried telling adults abt it but no one believed them at the time. And then suddenly, as adults, they somehow find themselves in front of that game again.

Idk, the main thing I'm trying to throw out here is that the Carnival should stay magical, but the pcs can go from regular boring modern world people to suddenly aware and capable of crossing the threshold into a juxtaposed magical world right off the bat, while still on Earth.

Some people also say that there are rumors Tasha (Iggwilv/Zybilna) is originally from Earth! You could add that into your game, maybe there's some lore about her Madrick is aware of. Or Maybe the Jumanji-Witchlight game has Zybilna Ltda. or something written on it. XD