There are no night trains between Berlin and Prague. Which is the connection you're looking at ?

ÖBB should be able to sell for up to 6 people but it is very well possible that night trains on 15th July are already sold out or have limited availability

Indeed, please mention departure time from Berlin. Or even better, post a screenshot of the connection details.

Day trains only take 4.5 hours.

I actually meant Berlin to Budapest (I have spent so long looking at websites and timetables I am losing my mind!).

There is availability but only when I book separately and not for all 5 together which isn’t ideal as we are travelling with children so don’t really want to be separated. Ideally would be good to speak to someone but the OBB number doesn’t work from the UK.  

Did you call the international number that can be found here?

https://www.oebb.at/en/reiseplanung-services/kundenservice/callcenter

15 July Berlin to Budapest only has seats available if you book for 5. Couchettes and sleepers are almost fully booked.

If you want to book seats, then what do you mean by "I can’t manage it for the five of us”? At what point do you get stuck? Is there an error? Could you post a screenshot?

In addition, remember to enter the children's age as 6 if you want them to have their own seats.

Alternative :

- Berlin Hbf - Praha hl.n. (Prague) 14:47 - 19:35 or 17:01 - 21:35

- Praha hl.n. - Budapest-Nyugati 22:03 - 08:29

1615 CZK (68€) for 5 couchettes in the same 6-people compartment through cd.cz (available right now, I've tried to book them and ended up in the same compartment). There is exactly 5 couchettes left, in the same compartment so book ASAP. It's quite cheap too :)

Thank you - I would have never worked this out. i will look into this option. 

I do have a back up itinerary that would involve a journey from Warsaw to Budapest - again a night train on Tuesday 18th July. But I am struggling to work out how to book this at all. Any help would be much appreciated.

That night train cannot be booked online. You can order reservations via Polrail.com though and you can then pick them up in Warsaw. Reservations are not open yet: Polish railways are often late when there's a timetable change (summer timetable comes into effect on 11 June). The advantage for you though is that nobody has been able to book yet. Not sure if there will be a couchette carriage (apart from seats and couchettes) but you could ask them.