Hi Wendy---like everyone else I will say you are moving too fat and have two many stops. Think in terms of nights at palace, not days. For four days in Paris, you have to spend five nights there. You will be traveling on Days One and Five. Even if you keep your rail journeys short, under four hours, it takes time track up, check out of the otel. Travel,to,the train station, and then reverse all that at the other end.
I am going to suggest a route that does not include everything you listed, but makes sense in terms of travel time and places most worth seeing. I know you are coming from Australia and want to " see it all" but if you try, you will be tired of traveling by your fourth stop. I did leave in Swiss Alps as that was on both your proposals, and is one of our daughters' favorites.
So after 9-10 nights in England, I suggest something like this:
Eurostar train to Paris, spend 5 nights ( look into apartments)
Train to Munich. This is a 6-hour journey and personally I would break that up with an overnight stop someplace charming. Strasbourg in Alsace would work.
Three-four nights Munich, with daytrip to the fairy-tale castle
Note that Oktoberfest begins September 20. Lodging prices nearly double, so you may want to avoid that.
Direct train Munich to Zurich, transfer tomLucerne, spend two nights.
Train to Interlaken and transfer to Lauterbrunnen, 20 minutes away. Much more in the mountains, and much nicer scenery. Spend 3-4 nights. You will want Swiss railpasses for the adults if you want to right the cablecars and Jungfrau train. The kids will ride free with a Family Card that comes with a pass.
Train to Milan to spend one night. This is a longish day but you will be busy making all the train changes. You go go via Lucerne and spend a night there to break up the journey. The Spiez-Brig route doesn't really have good overnight options unless younwant to detour 15 minutes from Brig to a little alpine village high above the valley, reached by cablecars. Bettmeralp or Fiescheralp.
Train Milan to Venice and spend three nights. This is a good time to see Venice, usually with nice weather.
Train to Florence for two nights. Train to Rome for whatever is left ( four nights, I think).
Use the German rail site, Bahn.de, to see the routes and schedules. It works for all of Europe. You will have to buy your tickets on the different country rail websites. Bahn.de offers great discounts for advance purchase on trains that originate or end in Germany. Use Trenitalia or Italo for the trains in Italy. You can investigate Swiss rail passes on this website.