I'd say:

1. Appropriate clothing for the conditions.
2. Water bottle with water or similar.
3. Whistle.


I think those are most important and can be carried and used by all but the youngest of children (with almost no training).

After those things the item list depends on the age and level of skill. One can carry all the gear they want, but if they don't know how to use it than it can actually have a negative effect on morale and safety.