Good call to get the night trains and Eurostar booked! That’s exactly what I’d recommend. The other trains are much easier, and you can decide on the day if you like.

It also sounds like a nice pace, without too much rushing around.

Day 2 - The direct TGV from Paris to Switzerland has quite expensive reservation fees. You can save a little by only taking the TGV as far as Strasbourg or Mulhouse, then taking a local train across the border. 

Day 4 - I think you mean Tirano to Milan

Day 5 - I don’t have any specific recommendations for campsites, but you have some great sightseeing nearby. Pisa itself is OK, but Tuscany, Cinque Terre and Florence are gorgeous, and all within striking distance. If you can find a base that allows you trips to one or more of those, you’re in a good position.

Day 8 - Again, Venice is gorgeous, and it would be a shame to miss it. Can you manage a one night stop in Venice into your itinerary? I’d strongly recommend the Meninger Hostel in Mestre. It’s a brand new hostel, high quality. And it’s almost on the station platform for the 10 minute commute into Venice.

Day 2 : direct TGVs to Zurich cost 29€ in 2nd class (39€ 1st class). Possible to save money by taking a TGV as far as Mulhouse or Strasbourg (10-20€) and then crossing the border on a regional train. Max 30 min longer.

Day 4 : you could spend the night around Lake Como (Varenna) instead of Milan. 

Day 8 : it depends on your preferences I guess. I loved Venice in winter but I've heard crowds are awful the rest of the year… maybe worth only a day trip and try to stay quite late.

Love your travel plan! We are also bringing our 10 and almost 8 year old this summer on their first Interrail trip. We are looking for a camping near Pisa as well and came across this: Camping Village Torre Pendente. Might be interesting.

BTW, we also plan to visit Venice just for the day, arrive early morning and take a late train out. I am curious how your final plan will look like :-)