First trip to Greece

For 2024?
Have you done any research into lodging availability? You don't have much lead time here. Lodging for four may dictate much of your itinerary. Use booking.com for research, then book directly.
Good luck!

Yes. I will be booking through Amex travel and I see good availability

You cannot reasonably visit Athens + Santorini + Naxos + Sifnos in just 10 or 12 days.

Santorini, except Akrotiri, does not represent any major archaeological interest and is not particularly suitable for a family stay with children aged 8/10.

Any other island we should consider adding/replacing

If you only stay in the Cyclades Islands, there are at least 15 other islands left to discover, by adding the other islands you have at least 150 others
I'm not going to list them. You will find it, at least partially, in any serious travel guide

With 11 nights if you can take a flight to Naxos as soon as you arrive in Athens, that leaves you 4 or 5 nights in Naxos, the Naxos→Sifnos ferry, 4 or 5 nights in Sifnos, the Sifnos→Piraeus ferry, and 2 or 3 nights in Athens at the end (or rather 3 nights for ancient Greece and mythology lovers)

Thank you! That’s very helpful advice.

I'm a teacher and a mom of same aged children as yours. I was just in Athens with a student group and I saw so many fun things to explore with kids. The Acropolis had many young Greek children visiting it on the same day we were, and the Acropolis museum has an amazing Lego display of the temples as well as a scavenger hunt focused on Athena. https://www.theacropolismuseum.gr/en/children-families/family-trails

The Plaka neighborhood in Athens had many interesting streets and shops that my students loved poking around. We didn't make it to the Archaeological Museum and did a brief stop at the Olympic Stadium. The ruins were interesting to students as were all the stray cats that they encountered at the various sights. Just a few ideas for the Athens portion of your trip. I'm dying to get back to Greece, as soon as possible.

We also took students to Delphi and Meteora, if those places are of interest to you.

Have you looked at Rick Steves suggested itineraries?

https://www.ricksteves.com/europe/greece/itinerary

I strongly disagree with RS’ choices of best islands.

In October 2018 my trip was 3 night in Nafplio with a trip to Epidzuros and Mycenae, 1 night on the ferry to Crete, one night in Heraklion, 1 night in Chania, 1 night on the ferry, 1 night in Delphi, 5 nights/4 days in Athens. There is enough to see on the mainland without needing to travel to islands. Maybe I would have been as happy if I had gone to Olympia in Western Greece instead of Crete. There may be a temple i didn't make time for at a place near Athens called Sounia.

I had a group tour in Athens that cost 7 Euros, led by a middle aged guy who grew up in Australia. Expensive guided tours seem like a rip off, if you could use guide books to find monuments on your own.

There may be other temples on the mainland you can see that I missed.

See if there are appealing sites in the Peleponnesian area, on the mainland west of Athens.

For interesting archeological sites look at the Peloponnesean Peninsula.