Nighttrain Paris-Berlin: how to book a seat?

I try to book 4 seats for my family for a trip between Paris and Berlin in late april.

However, I tried booking for 2 adults and 2 children (under 10) and the result is:

\- On sncf-connect (departure of the train), it is possible to book for adults and children older than 15, but not for the children with my age.

\- ÖBB, DB and trainline websites say that the booking is not open.

And it is not possible to connect either ÖBB or sncf-connect per phone to ask for the status of the train: bookable, full?

It seems that ÖBB sells the tickets up to 2 months before the departure (therefore, I would have to connect on late February) but I am worried, as it is a new connection and the spring, that all beds will be sold (and I don't want a simple seat especially for the children).

Do someone have some information on this subject? Thank you in advance!