In your case I would check if seats are left on Eurostar services for the day you want to travel. Because Eurostar have only a quota for pass users.
https://www.b-europe.com/EN/Booking/Pass#TravelWish (here you can check it without having a pass)
also keep in mind that kids under 12 have a pass for free, but need to pay for the reservations.
You have compulsory reservations on Eurostar (30 EUR), TGV Lyria Paris-Geneva/Basel/Zürich (38 EUR), EC Basel/Zürich/Geneva-Milan/Bologna/Venice (11 EUR) and Frecciarossa high speed trains in Italy (10 EUR). If you don’t take the direct trains on this routes you can avoid this high reservation cost of TGV Lyria and avoid completely the reservations for EC Switzerland-Italy.
Also possible is to take TGV services Paris-Milan for 35 Euro. They travel also via mountains at Chambery, Modane and Bardonecchia. is it scenic? I have no idea.
There are several routings, some of which have been listed already.
My choices would be either Home - London - Lille 1 night stop. Then Lille - Marseilles direct TGV - onto Nice for a night (or Cannes/Monaco/Menton. Then regional train to Ventimiglia to connect with direct train to Milan or Genoa, then Italy is all yours.
A second option would be the northern route - Lon - Brussels - Cologne - Frankfurt - Munich - Verona. Again a couple of nights needed.
Other than Eurostar both these routes have relatively low reservation fees - 30 euro per person should cover each way. (TGVs are 10 or 20 euro, The Brenner pass train from M nich has a 10 euro fee and Italian trains are either 3 (IC) or 10 euro (Freccia high speed).
Obviously you can go one way and back the other.
As you know prebought prices are rarely exchangeable or refundable and prices of turn up and go can be expensive.
As a guide the train from Lille to Marseilles has a typical price of between 60 and 138 euros depending on time and number of changes. A trip from Munich to Venice is from 60 euro (prepay non refundable) to 100 euro (standard flexible fare.
Just to add-or maybe to confusion:
also check if €* to BRU has more seats for you those days, it odten does as P is more wanted.
Check (only 3/week, every 2nd day ( NIGHT jet=overnite from BRU to München, or perhaps event hdaily DB-ICE seats only night iCE 619 from Köln-get there also on DB-ICE from BRU, to München-have a break for brekkie, then use the 10€ extra to pay direct EC trains via Brenner into Italy (Verona-Bologna/Milano). It may give you more opportunities and works out nearly as quick. Of course you could aso use this way to return and thus combine with via FR/CH.
