San Sebastián - How many days

My opinion is that’s a good amount of time to get a nice feel for the city.

3 nights for the area? or just for San Sebastian? With some beach, that is fine for San Sebastian, no real heavy hitting sights, just some interesting things to see, food, and beach.

Just be aware that it is a beach resort, and August can get crazy busy, we will be there in September this year and hotels were still filling up fast, and not many bargains...probably will be the most expensive rooms of the trip. With a car, you may have more flexibility where you stay though.

We are staying 2 days in bilbao and 3 days in San Sebastián. Got some airbnb for San Sebastian. Want to do day trips to French basque areas like St-jean-de-Luz, Bayonne. Any suggestions where else we can drive from San Sebastain. We are foodie people nad love villages or old towns.

The French Basque towns in the foothills of the Pyrenees above St-Jean de Luz are very charming-Saré, Ainhoa, Ascain, Espelette. They aren’t far from San Sebastián. We stayed in Hondarribia near the border of Spain with France.
You should see it too with a medieval quarter at the top of the hill and former Fishermen’s colorful houses on the street level.
I think you need one more night in this area.

San Sebastian is not a beach resort, is a working city that happens to have three beaches, but weather here is very unpredictable (and usually very rainy, SS is one of the rainiest cities in Spain), so do not expect anything like a beach resort, Obviously, in summer it gets crowded, as it´s generally nicer. Three days is more than enough, it´s a small city.

Well that really just two full days. To me, that would be good for just the town, so you'll have to be very selective with your day trips. What are the kids going to want to do? Play on the beach/go to amusement park, or ride around in the car all day?
Be sure your place has parking or they advise you on where to park.