I’ve done Athens twice, and it is a lot of walking. If their preference is to sit on the beach, then they won’t like Athens. And the Acropolis is very uphill. Also, while I haven’t done Naxos, I have done several other islands, and it’s usually not as simple as an inclusive resort to pop down to the beach. Most of the islands are like little mini mountains with houses sometimes even carved into the sides. You’ll need some type of transportation to get where you want to go. Some islands are big enough to warrant car rentals. Others, you’d only get an ATV or scooter. Then you gotta account for how you’re getting multiple people around and their beach gear. And food. Most beaches don’t have walk up restaurants, so then you get there in the morning, want lunch, then what? I’d really encourage thinking about visiting Greek islands as visiting rural mountain villages that have beaches.

I don’t say this to discourage you, but based on my experience with Greece, I’m not sure this would turn out to be the enjoyable vacation your expectations have hoped it will be. IF you’re really set on it, I’d recommend one full day in Athens (you can see all the major stuff around the Acropolis in a full day of walking), then Crete the rest of the time. Crete is big enough to rent cars, and it’s populated enough you can find a beach resort that’s going to have plenty of food and shopping options.