Sleeper from Paris to Barcelona - how do you book for a family?

[Mercedes]

We were thinking of getting the sleeper from Paris to Barcelona next summer (as recommended by SEAT61). There's me dp and dd aged 6 so we are going to need 3 berths.
When we go up to Scotland we usually get a family ticket that gives us adjoining cabins and a spare berth.
Has anybody any experience of travelling on the Paris to barcelona sleeper as a family and how this works out? Do we have to book an extra berth to get two adjoining cabins? If we book 3 berths will dp end up sharing with someone else?

no experience of the sleeper but loads of Barcelona if thats any help. Have you been before?

I was in Barcelona about 16 years ago so I assume its changed lots. Our plan was to have a beach holiday to the north of Barcelona and then spend a day or two in the city before getting the sleeper home.