I think that under 60 or so feet, privacy is an illusion no matter how many 'cabins' there might be. The key here is to have enough beds for everyone, and ideally to be able to do so without making up/breaking down a dinette arrangement everyday. That gets tiresome and can be problematic if someone wants to play some cards later into the night, or in other cases when someone else would rather sleep in than get up so breakfast can be put on the table.
We sailed (coastal) for some 10+ years on a 40 footer that was a 'crusified' racer - which still had 5 of the original pipeberths. 3 of them became kids' bunks - not especially private, but it was their own 'space' whether sleeping or reading on a passage or on the hook. This was a boat-partnership arrangement, and the 2 adult couples took turns with the vBerth and the dinette. Typically we would live for 3 weeks this way each summer, with weekends at other times.(so, if you're keeping track, that's 7, sometimes 8 bodies on a 40 footer for several weeks....) We're all still friends 10 years on.
Living with multiple souls in a confined space is definitely a learned skill/art - and it takes some effort to avoid getting on each other's nerves. Mind set and a good tolerance level will be as important as the 'layout' per se. Focusing the youngsters on the distractions and recreation available outside (swimming, kayaking, dinghy sailing, beachcombing, hiking, fishing, - who needs a Game Boy?) minimizes time below that might lead to feeling confined or penned in.
So you obviously need enough berths, but beyond that it's just a matter of making the situation work for everyone.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)