@Lollywillowes
congratulations on your new place, I'm sure you'll make it lovely!! (Sorry for how it's come about though!!)
Don't stress out about 'the stuff' today. It is what it is! You'll fit it in somehow!
How old are your kids?
Being a new build, it'll at least be clean (maybe a little dusty!) and no decorating needing to be done immediately!
so if it was me, I'd do what a pp did & put the kids in one room (if at all possible). & all their 'stuff' in another room.
depending on the house, stack the boxes in lesser used spaces garage/dining room one end of the lounge.
Work your way through them. It'll be a bit more tricky if you don't have bookcases/shelving storage. (What are your plans for this?)
would your Ex want any of the excess? I'm more thinking of an easy way to move stuff on, than what he wants.
I wouldn't pay for storage as its £££ and you have to go and collect the boxes etc. not so easy on your own, with kids. If the boxes are there you can potted when you get time.
put the kitchen stuff away, if you don't have room for everything & don't intend to buy more storage for in the kitchen then you can sort out what to keep & what to rehome as you go.
IF you don't think you'll be moving for a long time, don't keep things that you'll only use 'in another, bigger, house)
try to enjoy the last night in the old house
((HUGS)) it's not easy, msybe invite a goid friend around to keep you company & ward off the 'what ifs' x