Why don't you book directly via Interrail/Eurail ?

It is the best option, although there is a 2€ booking fee.

Also possible from SNCF or DB ticket offices or by phone from both (and some other railways). No booking fees in that case.

Ok, but how does it work via Interrail? IT does not work for me via website and via App: I can only book for one person, do I have to book every reservation separately (we are travelling as a family of five) and what if in the meantine the reservations are fully booked?

Your passes should have been added to the Interrail website. But if you book there (website only, not on the app), there’ll be 5 x 2€ booking fee (as with all reservations with Interrail, that’s why we advise to book somewhere else when available). Or simply call SNCF/DB (no booking fee) or visit a DB ticket counter if you live in Germany.

You’ll be seated together (either 2, 2 + 1 or a table of four and 1).

What do you mean by in the meantime ? Seat reservations won’t sell out instantly and you’ll buy them in one booking anyway. If you leave it to the last minute, you might not be seated together or the train might be full though.

Thanks for your help. I went to the DB ticket counter, but they couldn’t sell me reservations. So I tried it via interrail.eu but as I said it also wasn’t possible. I guess I found the error. My language was set to “German” and there is no information, that the booking period has not started yet.

I thought I was going to go mad because I couldn't find the seat booking. But when I changed the language to English, I got this very helpful information:

Could it be that this is why I could not book a reservation at the counter?

Yes you are way too early. Don't worry seats won't sell in a day anyway :)

As always, you'll get better information if you mention more details, most importantly route, date and departure time.