Might be an outlier here, but I'd actually steer younger kids away from the typical sprawling adventure retellings and toward something like 'Treasure of the Second Star' over on FFN. It's basically a series of short, self-contained chapters where Jake's crew solves small problems around Pirate Island—helping a baby dolphin, finding Cubby's lost map before a game, that sort of thing. The language is super simple, there's a gentle lesson about teamwork in each one, and it captures the show's bright, friendly vibe without adding any complex new villains or lore.
Honestly, for the real littles, you want fics that feel like lost episodes, not epic sagas. Another good one is 'Izzy's Sparkly Mistake,' where Izzy's pixie dust experiment goes slightly awry and turns Bucky pink. It's just silly, low-stakes fun with the characters helping each other fix it. The author clearly knows the audience; my nephew giggled through the whole thing. They're hard to search for because they're so short, but look for authors who tag their work 'for kids' or 'early reader.'