How old are the children?

11 and 9 years old

Your children are unfortunately too old for you to share a compartment for 3. In a couchette compartment you could all travel together. 

Are the couchette carriages back on this train?

If no couchettes are available then 2x2bed sleeper compartments would be the only way to get private accommodation for all 4. This will be €436

Yes, sleepers are not cheap, you have to pay for the compartments but still do get a significant reduction on the regular fare which would be €656.

The 2x2bed sleeper compartments don’t appear as an option available, the couchettes neither. The only option available seems to be the sleeper cabin for 2 or 3 beds (separate by gender). Screenshot attached:

That's the one you need to select. If you book the complete compartment, they're not gender-separated. If you book Double for all, then you'll have 2 complete compartments.

No thats why you still cant book it RDC suffer currently as they operate to many Nighttrains :/ 

Carriages for NJ Amsterdam - Zürich, Berlin -Stockholm + Couchette Prague - Zürich (as Ceske Drahy have no own carriages for that route)  happens recently that on the Nightjet Amsterdam - Zürich one carriage is missing aswell (often one of the carriages to Basel) 

More questions  In the web of cd.cz the train from Budapest to Stuttgart shows one transfer in Augsburg (screenshot one) but the Interrail web says it’s direct (second screenshot).

CD also says it's a direct train, in the option below the one with a change.

The best place to book is tickets.oebb.at - but not yet possible for that date.

Hi again: one more question about the train from Mannheim to Prague. I finally want to book  two sleeper cabin for 2 (separate by gender) in order to be me and our son in one compartment, and my wife and our daughter in other one (first screenshot). But when I’m gonna pay, this warning appears (second screenshot). I guess it means that we all four are not gonna be in the same compartment, that is gonna be two in one compartment, and two in the other one. Is that correct? It does’nt mean that, for example, my son is not gonna sleep in the same compartment as me, right?

I took this train recently, there are no couchettes available. 

Just a thought, your kids could probably be comfortable sharing one bed. Why not book one seat and one three-person-compartment and then just go in all four. 

Thank you for the idea. But it is possible? the train staff won't notice?

I can’t of course make any guarantees… But rarely are train staff so picky. Everybody has paid tickets and resevations for the train. And you are only using the space that you have paid for. So what’s the harm to anybody. Of course one of you wouldn’t be getting breakfast from the staff. 

You could get the seat for one of the adults and go there at first and then after ticket inspection go to your compartment. The very unlikely absolute worst case scenario would be one adult forced to sleep on the seat - can’t go worse than that.  

If anyone has any experience doing this, that would help of course. 

… I feel it’s the same as you had bought two separate  seats for you and your child and then the child prefers to sit in your lap. Who would make a fuzz over that…?

Thank you very much, Ikka. In any case, my question in particular is: why is the web telling me that we will be in separate compartments? I understand it’s because we are four people, and the compartments are with two beds, so we will have one compartment for two of us, and another one for the other two, right? But no each one of us in a different compartment. Is that correct?