For starters, do not get a Railpass. You can travel more cheaply on tp regular tickets, some bought in advance. Others you just buy there, like the ticket that covers 5 people on regional trains in Bavaria. Hopefully someone can explain that better.
Second, are you sure you want a night train? The EN train from Venice to Salzburg gets in around 4 am. What are you going to do then? Hang out in the station for 2 or 3 hours until someplace opens for breakfast? Not my idea of a nice trip. I tried to find prices for this and only came up,with around 258 euros on RailEurope, for second class seats. Is that a 6-person couchette, or really just seats? They don't say. Again, maybe someone can offer more help on finding a way to book a 6-person couchette and the price, but bottom line is I wouldn't advise taking the night train.
Besides, that route has you zigzagging up to Salzburg and back down to Switzerland and then back up to Germany your way to Amsterdam.
What I would suggest is this:
Buy tickets from Venice to Milan on Trenitalia. The trip is 2.75 hours and prices start at 19 euros for Super Economy fares ( book well in advance). Then book from Milan to Lucerne in Switzerland. You can book a Smart Fare on Trenitalia or a similar discount fare in the Swiss rail site, SBB.ch. when we did this a few years back we got the tickets from Zurich to Milan for 26 Swiss francs each.
Spend the night you would otherwise be on the night train in comfort in Lucerne. Next day head to Mürren, about a 3 hour journey including the cablecar up to the town.
Mürren to Munich will be a fairly long day, but it starts off fun with the cablecar. Buy tickets for the direct train Zurich to Munich in advance on SBB.ch or the German site, bahn.de. I see them as low as 49 euros for the direct train (bahn.de also shows a bus for 19 euros, taking the same four hours as the train, but it goes to Hackerbrucke).
From Munich continue to Salzburg and then Rhine area. Again, buy tickets in advance especially for the Amsterdam leg to get the best price.
You do not want a car for this type of trip because renting in one countryn( Italy) and dropping in another incurs big drop fees. Also, a car for five people plus luggage will be a van, expensive to rent and to fill with gas. Trains are great and will be fun for the kids ( and much more relaxing for you).