Thank you, Mary, for the detail. It is much clearer now. I wouldn't try to change your routing, but I do have some suggestions, which I think, taken in the round, may help you decide on the Eurailpass, or not.
The reason I asked about the ages was because the Belgian trains offer an absolute stonker of a senior fare, one cheap price for a round trip anywhere in Belgium, returning same day and starting after the rush hour in the morning, M-F, no such restriction on the weekends. But it sounds like Brussels will only be a stop on the way, not a base for exploring the country. Is that right? So those Belgian deals won't help you much.
If you are landing in Munich (?? or would it be another city for arrival ??) and heading off to Salzburg you won't need a Eurailpass, the really wonderful Bayern Ticket is built for groups and very cheap. It is valid on the regional trains to and including Salzburg, and in fact on every bus, train, tram, S-Bahn and U-Bahn in all of Bavaria. It does not cover local Austrian trains and does not cover transportation in Salzburg.
Because the train to Venice (Venice first ??) operates in Italy it would need reservation charges for Eurailpass riders, equally if you get advance purchases in for the train from Öbb (the Austrian train company) you can get reservations for all. The best train to me looks to be the 8:12 with just one change in Villach from an IC InterCity to a Railjet, which gets you in at 14:05, total travel time of under 6 hours. Only problem might be the 5 minute connection between platforms 3 and 4 at Villach which involved down and up the elevator or stairs. If you take a later train with a longer connection you pay for it with a 40 minute connection at Innsbruck and then a 32 minute connection at Verona, so you don't get to Venice until 16:42 with a journey of 7:46, nearly 2 hours slower. If you were doing that on Eurailpass you would need to buy and pay for 2 reservation fees for everybody because you would be on two different legs in Italy.
Italo trains tend to be a bit cheaper (and I prefer them) and make a good connection between Venice and Florence, but they don't accept Eurailpasses. Trenitalia does. Because the train to Florence from Venice originates Grand Canal side you can have a lot of time to settle everyone comfortably. I don't see any reason why one person couldn't buy all the pass holder reservations. A normal ticket on either Italo or Trenitalia can be bought on laptop or phone app, and include the reservations which will try to sit you together.
Cross border France to Switzerland or Germany or Italy are expensive. French passholder reservations vary in price, and are rationed, and you may find that while you could buy tickets on a particular train but the passholder reservations have sold out.
I wish I could be more positive, good luck, and I hope I have been a bit helpful