Florida Keys travel covers a lot of ground literally, about 100 miles of islands strung together by one highway. Marathon sits almost exactly in the middle, and that location does a lot of quiet work for families with kids in tow.
It means you're not stuck driving an hour just to reach a beach, a restaurant, or an attraction. It means grandparents who'd rather not spend the whole vacation in a car get a real break. And it means if half the group wants the energy of Key West for an afternoon while the other half wants to stay put and relax, both options are close enough to actually happen without blowing the whole day on a drive. Florida Keys weekly vacation rental
There's also Sombrero Beach, which deserves its own mention. Most of the Keys shoreline is mangrove, not sand beautiful, but not exactly what kids picture when you say "beach vacation." Marathon happens to have one of the few real sandy beaches in the entire chain, with calm, shallow water that's genuinely good for splashing around rather than just admiring from a distance.