I found this question on Quora.

Here’s my answer:

People can go through a lot or only a little change in appearance in 10+ years.

So instead of just recognizing them by sight, there are a few other things parents could do:

  • Four is an age where kids are fully aware of their world. If the parents bothered to tell them their full name, the child might remember. However, children forget a lot of stuff around that age.
  • Many districts put a baby’s footprint with its birth certificate.
  • DNA comparisons can tell with a high degree of confidence whether two biological parents are related to a child. For adopted parents, there is no way to use this.

It’s a good idea to collect photos, finger prints, and hair follicles with the roots still on before a child goes missing, so that police can catalog those things if an investigation becomes necessary.