A child under stress can forget a phone number they know perfectly well on an ordinary day. A card in a pocket does not have that problem.
Two phone numbers, not one — a parent and a second, out-of-town contact. A full name. An allergy or medical note if it matters. Nothing more; a crowded card is a card nobody reads under pressure.
An out-of-town contact who is often easier to reach if local lines are busy, and who everyone in the family calls to check in, is a genuinely useful backup - it costs nothing to add, and it is the one line on the card most families never fill in.
A shoelace tag, a zip pocket sewn into a coat, a lanyard under a shirt. The card is only useful if it survives the actual day it is needed, which usually involves running, rain, or both.
Yes - a stranger reading it on their behalf is exactly the situation the card is for. It is not for the child to read; it is for whoever is helping them.
Whenever a phone number changes, and otherwise once a year is a reasonable check.
Part of our guide to Emergency prep for families with kids.