Must haves:
camera
their first call (presentation ceremony recommended)
lots of fun snacks (pop tarts, crackers, Slim Jims, viennas, honey buns, snack cakes, candy bars, etc...)
lots of drinks for them (HINT: children don't usually like coffee or scotch)
Toilet paper
Heater of some sort, mine's a Coleman
extra clothing
recommended items:
GameBoy (they are kids after all)
"pottie" bucket
mp3 player
books
hand warmers
Activities:
Kids like running the jerk cord.
Tell them why you're doing what you're doing. Whether it is setting out dekes, calling on the corners, or whatever.
Teach 'em to call during the slow periods, and give them an example when the ducks show up. If they can make a quack or a drake whislte, LET THEM CALL!!!
Ritual's are fun for them too. We like to stop at a certain gas station and get our ritual breakfast, sausage biscuits and homemade cinnamon rolls for them and gravy biscuits for Dad.
Bottom line:
Make it fun for them and remember, you don't have to get a limit. My 3 girls had fun for 4 hours with the one teal I got that day. I think at one point they had tied a line around his neck and were trying to catch a gator.

I have a rule, we leave on the third time one of them asks to. Although, they like it so much, they are often upset when we have to leave.
Feel free to add to the list. Have a great season.