When I was a child, I managed to go events largely unsupervised and without a phone. I would already be told what to do in the event of getting separated in an emergency, such as having pre-arranged rendezvous points unique to my group.

> If they've run away from the venue without first unlocking their phone, they end up wandering around outside, at night, alone after a significant event such as this.

Only if their responsible adults don't take reasonable precautions. What happens when a child loses a phone or has one stolen? They should have a plan before going, whether phones are restricted or not.