Props. You want people to think like kids, get some props appropriate to their childhood ... pezz dispensers, sugary squeeze juice bottles, hotwheels cars, Shoots and Ladders, etc. Place these things around the gaming area for the players to fiddle with and look at ... then take the players out of their chairs and have them sit on the floor or on cushions/bean bag chairs while you run ... take away the things that they relate to adulthood - no cell phones or blackberries at the game - if they need something to busy their hands give them coloring books and crayons.
You might want to get some old cartoons online and run then through your computer or television ... though keep the volume low and, if you can, turn the monitor so it isn't distracting the game over much. If it is cold outside recommend they bring gloves and mittens ... maybe even buy some mittens at the dollar store - the same with hot cocoa to drink.
The important thing is: make the players feel like kids and their characters will follow.