Nowadays most people have a mobile phone. If they loose you whilst out and about, teaching your children your number could mean the difference between a quick call to arrange a rendezvous and a protracted trip to the police station to find them.
It’s not easy to memorise a nine digit number unless you have a motivation, so to teach our children mine I set it as my iPad lock code. I wrote it out for them so they could check it at first and within a week they had all learnt it.
To set it you need to turn off ‘simple Passcodes’ in Settings->Passcode (or TouchID and Passcode on a newer device).