I have a Jurgens Classique. It’s designed for 2, and reconfiguring it the way you are contemplating will be very difficult and will destroy the value.
There’s plenty of family caravans around which will easily accommodate your family, and they’re a whole lot cheaper than a Classique.
The way time flies your kids will be grown before you know it, and then you can enjoy your Classique the way it was meant to be. In the meantime get some cheap pop-top that will suit you admirably.