A new build sounds amazing to me, as someone who lives in a draughty, Edwardian house with wonky and bumpy walls, and an absolutely ancient boiler which the guy who comes to service it says will be unlikely to last the winter (he has said this for the last nine years as well).
Yours will have a lovely finish and everything will work. Your children will have their own rooms and it will be all new and clean and ever so exciting.
Please see the positives. There are many. This is a hugely exciting event for you all. There are amazing storage solutions available now to maximise space. Shelves above doorways etc.
Something that occurred to me that could make a difference to how much storage space there is in the bedrooms is how the doors open. I have no idea if it is possible to change them, or whether this contravenes any regulations or whatnot, but if they opened onto the landing, rather than into the rooms, you would be able to use the space behind them. Ditto the downstairs rooms into the hall.
Remember that you have an under stairs area and an attic for storage so if space happens to be tight in the bedrooms, you may have to be ruthless about putting away things that are seasonal only, keeping things like Christmas decorations that are used from year to year at a minimum (maybe embrace natural materials instead eg holly, twigs etc) and buy a decent sized shed.