Every trip between these cities is possible in a travel day. 

So if you'll do these cities in a circle you'd need 6 travel days (if going back to Marseille by train).

There's just a global pass for 5 or 7 days in a month. So you'd have a travel day left for a side trip or you may buy a regular ticket for the cheapest trip. 

You may stay in each city as long as you like to. You do just need to end your trip in a month from starting.

You do get free child passes for the kids when buying adult passes but you need to buy reservations for all. 

For Genf to München, München to Praha and Praha to Wien there's no reservation needed (but possible if you'd like to reserve a seat).

You'll just need to buy reservations for the other trips. For Wien to Venezia, there's also a daily night train.

Apart of this, you do just add the trains you are travelling with to your rail planner app (when buying a mobile pass) or write them down (when buying a paper pass) before boarding. There's no need to book anything if the train has no mandatory reservation.