Italy-SOF-Paris? (HELP Please!)

when are you losing the Venice car?

Yes take a day from Lake Garda and add to the SOF. I assume you will have a car for the France bit. If you take a day (I would even take two) from Garda that means you will have 5 or 6 nights in France. I would do two nights in Aix (one to see that city, one for the day trip to Cassis (or a winery) and then move to St Remy for the remaining nights, during which time you would do Avignon as a day trip. You could also do Arles and other towns on day trips.

Thanks! I was trying to keep from too many changes of accoms because of the kids. However, we will have a car from the time we leave Venice until we depart SOF for Paris. So that shouldn't be too much a problem.

Have you priced rental car yet?
Drop off fees in different country can be quite high and car rentals are already super high this year.

I've got to get my head around this SOF - an expression I'd never heard before.

If you have a car rented in Venice and want to return it anywhere in France, south or otherwise, you'll be paying a large drop charge on top of the high this year rental charges. I have seen it as high as many hundreds of Euro. They can't rent that car in France - it has to be returned to Italy by an employee, and people in that situation expect to pay.

You can take a direct train from the Venice S. Lucia station to Vicenza (45-minutes) and rent a car in Vicenza and drive to Garda. To get from Garda to Aix drop off the car at Milan’s Bergamo (BGY) Airport and take a nonstop flight to Marseilles and sleep there two nights. The next day you can rent a car or take a direct train from the Marseille Blancarde station to Cassis (30-minutes).
After two nights in Marseille you can take a direct train from the Marseille St Charles station to Aix En Provence (45-minutes).

If you find a car rental agency in Italy that allows you to return the car in France, it will cost you a real fortune.

Venice with little kids doesn't sound fun and especially not for more than two days. I'd skip Italy and just do France. In Paris stay near Luxembourg gardens which is great with kids. Take the Tgv to Avignon,rent a car a stay in st. Remy. Be sure to check out the point DU gard and bring bathing suits. The Dordogne is another option which is kind friendly. Don't underestimate the time it takes to travel between France and Italy, it's a plane flight or twelve hours of trains. Driving a rental car is a bad option.

Logistics from Northern Italy to the Marseille area are difficult. There is a very expensive flight from Milan (Malpensa) to Marseille (or at least there was pre-covid), and that's about it. The destinations do not combine well!
Perhaps look at the Ligurian coast instead, between Genoa and La Spezia?

Edited: MaryPat points out below that Ryanair flies from Bergamo to Marseille. It is not daily, but it would be the only affordable way to combine Lake Garda and Aix.

With the best of conditions, the road trip from Garda to Aix will be over an 8 hour day. How will your 3 and 4 year olds handle the car seats for that long a time? Perhaps you should look at basing further east along the Côte d’Azur for some beach time with the kids, and trips up into the hill towns. Many people take several trips to fully experience the Provence/Côte d’Azur region.

If Provence is really your priority, I would still consider a night or two in the Nice-Antibes area to break the journey. Then transfer to Aix, stopping in Cassis on the way.

Are you planning to take the TGV to Paris from Aix?

There is a very expensive flight from Milan (Malpensa) to Marseille (or at least there was pre-covid), and that's about it. The destinations do not combine well!

Ryanair has a nonstop flight from Milan's BGY to Marseille for $13 on Jun 3. There are no nonstop flights from Genoa or La Spezia to Marseille.

I did not know about that Ryanair flight! It's not daily, but at least it's an option (better than that 300€ Malpensa-Marseille flight on a wobbly 19-seater plane...).
And to clarify, I mentioned the Ligurian coast as a substitute for Provence - not as a stopping point.