Consider these things before you say that it was all a childhood fantasy:
All the things that exist in this world, all the dialogs that are sometimes so complicated you feel really stupid (remember when Toothrot asks about the tree that falls when nobody is around to hear it?) may take more than an eight-year old to make up, imho.
When Guybrush walks into the carnival (of the damned) at the end as a child, he says something like "what's going on here?". That's something you would say if you felt something isn't right here.
Chuckie makes evil eyes when walking, very high-class hint that this isn't a normal family.
Don't forget Elaine that wonders what takes so long and hopes that LeChuck didn't put an evil spell on Guybrush!
I think that it was a spell cast on Guybrush by LeChuck, which he does once again in MI3. Guybrush then managed to break the spell and escape in a bumper car, also in MI3 he breaks the spell but there you have to do it yourself. Exactly when the spell is cast is a mystery I think, it may have been when LeChuck sends Guybrush to the dimension of infinite pain, because that's the only part when something magically really happens to Guybrush. Then in the other world it's also a question if it's LeChuck that plays Chuckie or if it's one of LeChucks companions, because in MI3 LeChuck asks Guybrush how he escaped from his carnival of the damned so obviously he hasn't had any oversight of Guybrush.
Then there's the ticket, I don't really remember if the ticket shows up before or after LeChuck sends Guybrush away, but either way the treasure is some sort of ride ticket and correct me if I'm wrong but didn't LeChuck say that Big Whoop is a gate to hell? Maybe it's a ride on his carnival that requires that ticket.