Last night in Paris for Family of Four (18+)

You are correct about the car tour. According to the drop down menu on their booking site https://2cvparistour.com/ three is the maximum number of passengers you can choose so you’d need to book 2 cars.

An alternative is certainly a Seine boat cruise and we’ve done this several times just by showing up and buying a ticket. We’ve used both of these companies.

https://www.vedettesdupontneuf.com/home/

https://www.bateaux-mouches.fr/

A couple of my colleagues (who already know Paris well) recently enjoyed a Deux Chevaux tour with a talkative and friendly guide. But if you want to try with a regular taxi or uber that's big enough to fit your group, then Rick's Paris 2019 (the bigger one, but not the France book) describes a route you can request from them (from p. 506).

I've enjoyed a river cruise like the Vedettes de Paris, departing from the Eiffel tour area, with outdoor seating, and not a dinner cruise. Other companies might be more convenient depending on your hotel location, etc.

We never took a night tour on land, but the two most memorable evening activities our family did in Paris were a river cruise and going up the Eiffel Tower at dusk, coming down and leaving as the lights came on.