Bookings are released in batches because SNCF. Dates from 23rd May will be opened on 7th March. See : https://www.sncf-connect.com/aide/l-ouverture-des-ventes

Book through Rail Europe to avoid fees, have a look at : https://www.seat61.com/interrail-and-eurail-reservations.htm

You'll have to do multiple bookings (one per family so at least they're sitting together) or call SNCF : https://www.sncf-voyageurs.com/en/customer-service/contact-us/telephone/

Feedback appreciated. :)

This is extremely helpful thank you. If we wait until 7 March to book the return, do you have a sense of whether there would likely still be plenty seats available then? I'm wondering how quickly they sell out in holidays. 

Trains never sell put immediately anyway, no to worry about that. If you make the reservations together, you'll be sitting close to each other.

Did you also check group tickets? Another way to ensure you'll be sitting together (but only if you haven't bought Interrail/Eurail passes already. https://www.sncf-voyageurs.com/en/travel-with-us/plan-your-journey/group-travel/travel-with-group-10-60-people/

Nobody will be able to book earlier than 7th March (whether tickets or passes) so I don't really understand the question.

It's not a holiday either, not in France at least.

All v helpful, many thanks everyone