Hi Lindsay
dont leave everyting till the last minute , try and load the van a few days before hand if not the previous weekend
(we've gone to the expense of having clothes specifically for the van that we leave in during the season).
Do the safety checks (i.e tyre pressures) the day before if you can.
Try and leave the day of travel for those last items you cant put in the van earlier and for actually hooking up the car and van ready for set off.
You dont say how old the kids are but get them involved as much as you can like packing their own little rucksack with toys (make it a game) Obviously make sure you dont forget favourites!!!!
On the day of travel DONT set a too ridgid set off time or be to expectant on time of arrival its just too stressful , go with the flow the journey is part of the holiday.
Lastly , drive a short distance , then pullover and do an once round the van to make sure everythings as it should be.
I once left an outside locker open because I'd got too much swirling around and forgot about it as I was in a rush.
Dont rush
You dont say where you are going? It might be worth planing ahead a looking for ways to entertain the kids , if the weather is bad and expect the weather to be bad this time of year so pack the wellies and kags'
Go luck , most of all enjoy the experience and have a laugh