Naxos or Sifnos with young children

GI ven that you are starting late with booking, I would suggest going to Sifnos & seeking lodging at PLlaty Gialos beach or Vathy beach. Both are sandy, and are shallow for the first few feet into the wataer... many pre-schoolers just sit at waters edge with pail & shovel.

Naxos' St. George beach, right next to town, is Bullseye for pre-schoolers, sandy & v shallow until far far out.... however, Naxos alas has been "discovered" and unless you go in late May, I fear for the crowds these days. Sifnos does not have an airport, which lessens the tourist impact. It does mean that you must fly in then taxi to Piraeus ... the last ferries to Sifnos are about 3:30 pm. For least stress/most expertise in getting suiitable Sifnos lodgings for familly, I suggest you contact immeidately Aegean-Thesaurus Agency. It is experienced with serving N. American clilents online, serves all of Greece and has an Athens office --- but it Began in Sifnos & knows it intimately, will give great service there... and also Athens/Nafplio.

I hope you already have bookings in Aathens & nafplio -- Greece tourism starts earlier each year! When u contactd A-T, don't be general or vague --- give exact dates AND time of day for your plane arrival/departure, the exact dates u want for each of your 3 destinations, your room requirements and your budget range per hight. This is how you get results!

I'm guessing you will rent car for your Nafplio time.... for beaches, the longest sandy beach is TOLO, about a 15 min drive from Nafpilio Old Town... The beach is Ringed with hundreds of hotels/apts/pensions, so will be busy, but not jammed before july. My fave "secret beach" is about 2 miles away --- Kastraki, across from the Hellenic ruins/wall of Asine. (look at map). It has a tiny rocky cove, and a small sandy cove... looks right across the bay at TOLO, but is lovelly & quiet. Oonly one taverna there... and you can use their restroom to change, when you buy snacks & beverages. Don't tell anybody.

Janet, thanks for the suggestions. We are planning for 2025 (see above), not this year.

And edited to ask: How likely are ferries between Piraeus and Sifnos to be affected by weather in mid-June? And yes, we would rent a car for Napflio.

I agree with Janet about choosing Sifnos over Naxos, especially as Sifnos becomes more and more popular every year, go there before it becomes overrun with tourists from June to September just like Naxos or Paros.

See the Sifnos website:

https://sifnos.e-sifnos.com/

I also agree about Aegean Thesaurus Travel, a travel agency in Sifnos which has existed for a very long time and which found accommodation and car/scooter rentals for visitors before the internet era.

https://www.thesaurus.gr/

That being said, you can also use the usual services like booking.com or Airbnb to find accommodation.

If you don't want to go through an intermediary see "rooms in Sifnos" a locally managed website where you can contact the owners directly

https://www.roomsinsifnos.com/

Almost all the beaches are suitable for young children like those of Platis Gialos and Vathy mentioned by Janet, I would also add Faros which is less busy but in the same area.

I do not recommend Kamares, the port which, although having a beach which may be suitable, is in the North and is in my opinion a place in Sifnos for people who do not want to discover Sifnos.
Sifnos is also famous for its gastronomy, which is also why as foodies we will probably be there next September for the Cycladic Gastronomy Festival :))

How likely are ferries between Piraeus and Sifnos to be affected by
weather in mid-June?

Although bad weather preventing ferries from sailing is infrequent there is always a risk, whether in June or any other month.
Anyway you will only know the weather forecast 3 or 4 days before the trip. So no need to worry about it 15 months in advance.

Thank JoLui! And thanks for your advice earlier this year on France. We had a great trip.