Mind the heat

The Prime Minister also issued a warning of a long summer of wildfires

https://www.independent.co.uk/travel/news-and-advice/greece-holiday-flights-fires-heat-b2571774.html

oh no! This makes me rethink our Greek islands trip Aug 20-Sept 3. Where else in Europe would be a good pivot for a low key, beach-centered family vacation? (we love history and ancient ruins too)

You can read the same kind of article every day in all the European press.

All of southern Europe, Spain, the Canaries, France, Portugal, Italy, Greece, and even England are subject to an increase in unpredictable extreme weather phenomena such as heatwaves, water shortages, wildfires, coastal erosion, floods, landslide, causing evacuations of entire regions, water shortages, transport cancellations...

And regarding beaches, note that climate change also favors the spread of venomous marine species such as jellyfish

Where else in Europe would be a good pivot for a low key, beach-centered family vacation?

If it's beach centered, I don't know why you'd change. You're not going to get headstroke if you're plunging into cool water every so often. The warnings are for people who are doing a lot of sightseeing, especially at the hottest parts of the day. I remember feeling like absolute garbage when my husband decided we should go traipsing around ruins on Sardinia that were literally next to huge sandy beaches. My suggestion had been that we beach first, and explore after 17:00. Instead we were in the worst of it at 13:00 and I've barely forgiven him.

But low-key, beachy? You'll be fine in Greece.

You definitely need to be cautious during summer in the Mediterranean. Some tourists who went missing had the wild idea of hiking in the afternoon, which is really risky. Even locals in Greece take it easy between 12-4 PM! So, it’s better to enjoy a siesta during that time while you’re in Greece.
Also, if you’re more of a swimming pool person, consider visiting Greece between October and November. In my opinion, it’s the best time of year with no crowds, fantastic weather, and plenty of options available.

I am planning a trip for end of August to Chania, Crete. Does anyone know how the wildfires are at that time? I did get some historical data about July but not able to find out how things are in the last week of August i.e. what is the risk of wildfires then. Is it advisable to go later in September or October to minimize inconvenience due to any such incidents?

No one can predict when or where a wildfire will spring up. But it will be slightly less hot in September, and there is more chance of precipitation. In October even more so