Very Last-minute France with teens late April

For such a short trip, I would use an airport that you can fly directly to/from. If that is Marseille, I'd choose that. Get a car. Base in Arles (or wherever you can get a suitable place between Arles and Aix). Day trips to Avignon, the Camargue, etc. For a family with teens, I always recommend AirBnB or a gite, but you might need to take what you can get at this late stage.
Have a great time!

Given everything you have said, either Marseille or Nice are good options. With seven nights, I would personally pick either Provence or the Riviera. We found the Riviera to be generally more expensive, so if the trappings of luxury aren't for you, I would bias a bit more towards Provence. Conversely, if Grasse is high on the list, Nice is the easier choice.

For your beautiful but manageable hiking, Cassis and the Clanques are terrific (Provence). Verdon is another option (a bit easier from Nice).

Provence gives you significantly more/better Roman ruins, Van Gogh, medieval religious history, lavender fields, etc. Both will provide hilltop villages. The Riviera obviously gives you the sea, but Cassis is a great proxy for this if you focus on Provence. Nice wins for museums and shopping.

If it doesn’t work out for this trip, I do highly recommend visiting Normandy as a family sometime if you have a personal connection. It was really meaningful for us! We also visited Anzio in Italy where my grandfather-in-law was stationed, and being able to experience that together with my father-in-law was really special.

Decided to fly to Nice, booked the tix, rented a car, will have 7 nights and 8 days. Quite interested in Cassis and Provence, more so than glitzier Cote-d'Azur towns. Does that narrow down the recommendations?

Cassis is about a 2-hour drive from Nice. Assuming this is both your arrival and departure airport, and that you are hitting the road when you land, I would spend 2 nights in Cassis, then move to a more central location for 4 nights like Arles, Saint Remy, or Avignon. Aix is also often recommended but is a bit further east of some important sites (depending on your interests). I would then probably trek back to Nice for a night to catch your flight home, especially if a morning departure. You could hit Grasse on this day.

Not everyone likes jumping in a car after a long flight, so you could instead spend day 1 in Grasse and head out for some version of this itinerary in the morning. Maybe reverse the order and hit Cassis at the end of the trip.

smac68,
Just a reminder, seven nights means six full days. Your arrival and departure days are truncated due to arrival and departure necessities. Arrival day is immigration, possibly customs, picking up rental car, getting to first night's lodging, and maybe facing jet lag. Departure day is checking out of lodgings, getting to airport, gassing up and returning car, security and checking in at airport. Be aware that your first and last days' plans should take this into consideration, wherever you decide to stay. A lot depends on your arrival and departure times.
You will all have a wonderful time. Amusez-vous ien!

I visited Nice for the first time last Sept and going back again this Sept. I can't imagine ever flying into Nice without spending at least a couple nights there. But to each their own. Can't comment on the rest as have never been anywhere else south.

Part of the charm of Nice is all the beautiful towns and cities that are a short train or bus away.