First, go to http://www.aikidokids.com
for lots of game and curriculum ideas.
5-year-olds do indeed learn differently from adults. Children vary widely in their focus and attention span, but generally speaking:
-35-45 minute class length
-activities change every 8 to 12 minutes
-almost any skill drill can be a game
-do teach and expect your standard dojo etiquette. We began class by lining up at the edge of the mat to practice bowing on and lining up, then bowing in formally
-I rely a lot on the aiki taiso movements; they provide a good framework of skills and lend themselves to partner variations
-with 6 young ones, 4 & 5 years old, I found I needed another adult to do any partner practice
-they're little sticklers for routine; don't be afraid to repeat the same class several times before introducing one or two new things.