My younger two were 10 and 13. We arrived in Athens at 4p and drove to Nafplio from there. We did stop at the Corinth Canal (got off the freeway and drove to the little bridge you can walk over). I would definitely do this. The kids liked it a lot. So we got to Nafplio at night, and then spent two full days there (3 nights). Our first day we went to Epidauros in the morning, and Mycenae in the afternoon (driving straight from Epidauros). I figured Id see how everyone felt after Epidauros, but we were all game. Everyone liked Mycenae a lot. The driving (there is a LOT of driving in Greece; one of my complaints) provides a break from walking, but also motivates the kids to want to explore, no matter what you will be seeing. We all found Mycenae easy to replicate in our imaginations. The view is incredible, and we could tell the kids the story of it being an outlook and of Agamemnon, and they could really 'see' it. I think the fact that it is a ruin, with trails leading from part to part, made it like a maze for the younger two to explore (they took off ahead and explored on their own - and found a deep cave that they then had fun leading the rest of us down). We finished at the museum, which for us, is the hard part for the younger two: museums.
The second day in Nafplio we climbed the Palmidi fortress in the morning (not challenging, great views!, not as interesting as Mycenae), and did some other activity in the afternoon.
One thing my youngest enjoyed during the trip was counting all the cats. She kept a running tally for the month, starting in Nafplio, and ended up counting 210+. Nafplio had a ton of cats, maybe more than other places, so maybe the 7 year old would like to start a tally
Naxos was great because there was more to do than just a tourist center (Chora)/port and beach.
This was our theme - we prefer to experience the everyday life of a place, and not just the tourist hits. So although we do make a point of experiencing the highlights, we then head into les charted places if possible. In Naxos, my favorite thing was driving into the mountains, specifically, the Byzantine churches outside of Halki. You walk along a trail through groves and in the middle of them, you stumble on a gorgeous, intimate stone church with outstanding frescoes, and then further along the trail there is another one. The churches are open for viewing only during limited hours, and as we literally stumbled on them ("this sign says theres a church down here, lets follow this trail...") we were lucky to show up just as the volunteers who staff the churches showed up to open them. Then we drove the Apeiranthos, which was beautiful, but another 'highlight', so had that touristy feel I dont like. The drive there and back was spectacular (we took a different route back). We also made a point of staying well south of the Chora, and only went in to walk around for an afternoon. However, we were there during the high season, so you will not have the hordes of tourists. We liked Naxos because it was beautiful and so diverse in its geography, and had so much to do (hiking, biking, cliff jumping, beach, town, etc). Whereas Santorini was focused on tourists - staffed by people and services for the tourists. It was very beautiful, but really the cruise-ship-effect combined with the 20-something party people effect was just not for us. That said, again, you will be there in March (which could affect you negatively perhaps bc of lack of services? I dont know)
We loved Athens. I read here that its not very pretty and not a lot to do and no need to spend a lot of time. We spent 2 days and could have stayed a third. I think by the end we were so sick of beaches and venetian ports and wanted city life and museums. And the Parthenon was gob-smacking (my husband said its his favorite ruin, better than the Colosseum), and the kids loved (!) the Acropolis Museum. We did not go to Meteora