My son is 6 and has had an EDC since he was 4. It goes on/in the pack he takes to school. Most likely your son also has a couple of packs so it has to be quickly portable, (i.e. the wheeled case, the Ironman backpack, the old pack of mom/dads)

As mentioned, 90% of time he is chaperoned. So the most important to us is the whistle and photon light which are somehow on the outside of all his bags and brightly colored. On his backpacks they're tied to the shoulder strap and fastek on/off so he can get to them without taking the pack off--KEY FOR THE WHISTLE. A tiny anti-bac is strapped to the outside too.

He has zippered pouch with some items for his glasses, a couple of bandaids, couple pieces of hard candy, a non-melting nutrition bar that he likes, capri sun, sunscreen he can put on his face himself, a small toy (which he takes daily), camo bandanna (and trained to soak with water for hot days, put over mouth for dust storms or windy days), pair of socks (cause look he has a lot of sand on his playground)