I don’t think night trains make a lot of sense for this itinerary. Paris-Amsterdam-Brussels are 2 or 3 hour train rides. Austria (assuming Vienna or Salzburg) to Venice is the only place where I’d take an overnight. Brussels-Nice requires departure from Brussels Midi station which, depending on your hotel location, may mean a transfer in Brussels. Next there’s a transfer in Paris which requires a subway ride between Nord and Austerlitz stations and would use 2 days of the a flexipass. The option that only requires 1 day of a flexipass (which is probably the best deal financially for this itinerary) involves a 7 hour layover in Lyon between midnight and 7 AM — sounds like fun with kids to me. I’d check into overnight trains between Berlin and Krakow and Krakow and Prague. Something might work there but you might find a cheap flight works better. Venice to Monaco and Monaco to Spain are both routes where I’d definitely be looking for a flight.

I’m not bothered by the train noise on an overnight, but I am bothered by the stopping — most people wake up every time the train stops. Passenger trains have priority all day. Freight trains have priority at night and you spend a lot of time parked on a siding waiting for a freight train to go through. My rule of thumb is to average no more than 1 overnight train per week and never take more than 2 overnight trains in a 4 day span.