Note: Marieke and I got divorced in 2020. This post might not reflect the current situation.
Rather than opting for labeling all their clothes, giving them a leash or giving them some sort of engraved bracelet, we opted for a song, the phone number song. A really simple way to teach our daughter Ella (and later, our son Gust) what to do when they would get lost and would find a stranger on the street. The “phone number song”. Every day, for several weeks, my wife would practice the phone song, which would go something like this: Zero-Six-One-Five-Four-Three-Six-Four-Three-Seven (set to a melody). We taught our kids that when they would get lost, they would have to sing the song. That would help people get our phone number and contact us.
Everything is easier with a song. The phone number song in this case.