Vacuum seal cold meats from the deli (ham, pork, turkey, chicken etc), get a Shed load of prawns on Xmas eve, salads and a Xmas pud with brandy custard. Make sure there is enough alcohol and you'll be set.
Take a couple of small gifts for the kids and tell them Santa delivered the rest to your house. That way the kids get something to open when they get home to take their minds off the post camping blues (works for mine)
Decorate the camp site with solar lights, tinsel etc. the kids won't notice anything different.
Xmas is about family being together, I think you guys will have one of your best yet, you can't beat camping at Xmas.