I would think thermal regulation and hydration is of primary concern.
Therefore, I would gear the kit to accomplish this task.
It would also seem that a survival kit hung around the neck, would be very small. A kit this small would require more expertise than a slightly larger kit. I would suggest a small backpack or fanny pack.

1. A small pill bottle filled with petroleum jelly treated
cotton balls.
2. Ferrocerium rod and striker.
3. Mora 511 knife (if the children are competent and trained to
use one).
4. A bic or cricket lighter.
5. A small, easy to read pamphlet with survival tips.
6. Space blanket.
7. Contractor size trash bag.
8. 20 feet of #36 bankline.
9. A candle.
10. Small first aid kit.
11. Water purification tablets.
12. Small metal container for melting snow.
13. 1-liter bag to carry water.
14. High calorie/protein bar.
15. Whistle.

If the kit has to be worn around the neck, then items 1,2,4,6,8,10,11,13,14 and 15 could be carried around the neck.