Almost nobody knows a telephone number by heart any more. This is a genuine change in how emergencies go wrong.
Write down five numbers. The out-of-area contact, one relative, one neighbour, the school, and your own number so somebody can read it back to you. On paper, in every bag and wallet, and in a child's school bag.
Emergency numbers differ by country. 112 works across the European Union, and several countries also run their own numbers which reach different services. If you travel, or if you have family in another country, check that country's page on this site rather than assuming.
Teach children the out-of-area number, not yours. Yours may be unreachable precisely when they need it.
A card in a young child's bag with their name, a parent's name, and the contact number solves an enormous problem for whoever finds them. Not their address — a name and a number.