David, I don't know the exact layout of the caravan you are thinking of, so I cannot comment on its suitability. However, the usual drawback of fixed bed vans for families, or at least for those with younger children, seems to be that once the children are in bed, in beds made up from the seats, there is nowhere left for the adults to sit and relax. This may not be a problem in warmer weather, when you will probably be happy to sit outside, or in the awning, but outside of the summer months, you might well find you are forced to take to that lovely fixed double much earlier in the evening than you would like, simply because there is nowhere left to sit!
It is largely for this reason that we have continued to prefer the double dinette arrangement, which still seems to offer greater flexibility, if not quite the same level of comfort as can be found with a fixed bed. We normally caravan as a couple, but sometimes take our grandchildren, or lend the 'van to our children and their families, and the layout that served us so well when we had a young family still seems to best meet our needs today.