Tuscany or Venice - how much time?

Two nights in Venice should be the minimum, IMO.
On the other hand you seem to have a very limited number of nights, and too many things you want to see. A common problem to most American travelers.
How about asking your Congress Representative to pass a law to require companies to give at least 7 weeks of vacation a year?
For now, you’ll have to cope with the nights you have. One night in Venice is better than none.

If Venice is a focus I might recommend that you venture out around Venice into the area as there are some lovely places that will allow you to "slow down". Tuscany will be there for your next trip.

Stick with Rome and Venice for this trip.
You will hopefully see hilltop towns while in Provence if not make a point to since you really don't have time for Tuscany one's on this trip

4 nights Rome, 3 nights Venice would be a much better trip
You want to slow down a bit but with only 3 nights in Rome you will be racing around Rome to see those sites and 1 night in Venice which is just not enough same thing the whole time will be rushed.

Where do you fly out of to go back to Paris? How much time are you allowing to get to your departure?

If your goal is to slow down the pace of your trip, leave Venice for another time. You list 3 nights in Tuscany, so I'm assuming:
Day 1 -Travel from Orvieto to Agriturismo - There's a lovely drive up into Tuscany from Orvieto. You could hit Montepulciano, Montalcino and end up near Siena. Look for an agriturismo near Siena.
Night 1 - Tuscany
Day2 - Visit Siena
Night 2 - Tuscany
Day 3 - Either head to San Gimignano and Volterra or go up SR222 to Castellina - Radda - Greve.
Night 3 - Last night in Tuscany.
If you drop Venice, then you could do whichever you didn't do Day 3.

Venice - 1 night - is 1 night enough

No

is it a waste of time if we can only do 1 night?

No

Its kind of like having tickets to the Super Bowl, but having to leave at the end of the 1st quarter.

If you have to be in Venice for your flight to Paris, I would do 2 nights Florence, 2 nights Venice. If you have the option to fly Florence to Paris, and are looking at doing 'slow down' at the end of your trip, the 4 nights Rome and 3 in Florence.

It all depends on what you want.

There is no 'wrong' choice.

Since you "all" want to see Venice--go there!
Spend your 3 nights in Rome, since you are already booked. Can you add 1 more? If not, then just go to Venice for 4 nights, and do day trips.
Your son might like https://rowvenice.org He can learn to row a gondola.

After Rome, you could spend a night in Orvieto, then go on to Venice by train.

We enjoyed Agriturismo Marciano outside Siena. Easy (maybe 1 hour or so) train ride to/from Florence. My mom and I actually took the bus from Rome to Siena, then got a short taxi ride to the Agriturismo. Loved it there so much, I am bringing my husband and 14 year-old son back there this November for three nights! We will rent a car and stay there 3 nights. The plan is to use it as a base for day trips through Tuscany, etc.
I think one night in Venice could be enough if you have 2 days there.

Thank you all for your thoughtful responses!