I'm totally gonna create art for this - already have - but in the meantime I should probably explain this concept lol.
So basically, in one way or another, the turtles each stumble upon the Hidden City at around age 8 to 10, and decide to keep it a secret from their bros >:3
Leo is the first one, technically, when he goes out to explore NYC when he's eight, and discovers Run of the Mill Pizza, because I'm obsessed with the Uncle Hueso idea, but he doesn't go to the actual city until much later, though he knows of it's existence.
I actually kinda started writing this starting bit? It's totally unedited and missing bits but whatever, I'll include it under the cut.