There is a lot of good information at http://www.equipped.org/

... including a Survival For Kids link http://www.equipped.org/kidsrvl.htm

... which has a Kid's Don't-Leave-Home-Without-It Equipment link that recommends the following items:

Identification and/or Medical Alert Tag or Bracelet
A Loud Whistle
1 or 2 Large Garbage Bags (Orange! Big enough to cover the entire body, with another to sit on)
1 or 2 Canteens of Water
A Pocket Flashlight
A Brightly Colored Bandanna (Orange!)
Cell Phone
If you know how to use them properly and safely:
Pocket Knife (a sturdy locking folding knife is best)
Firestarter (matches, lighter or flint and steel ... and add some good tinder too, such as petroleum jelly on cotton balls)