I can’t speak for other countries, only the United States.
A child can get lost in the system, fall through the cracks, and nobody will notice until is too late. Everybody wants to help a child, but when we fail that child, there is no one to be seen anywhere near the child. That was worth mentioning.
Unless the child is born from a mother whom nobody knew was pregnant, that child will always be identified. Children are assigned a social security number at birth that tracks them until the day they die. Even a prom baby can be identified via a DNA process. This is the same diagnostic tool that is used for a “found” infant or child, usually in conjunction with authorities who run searches for missing children. Missing children who are never found are either dead, or an adult has absconded out of the country with them.
You are legal required to enroll a child in school. People will notice, believe me, if you have a child who is not attending school when it is in session. An uneducated child is glaringly obvious, so even if a person doesn’t quite know what to make of the situation, social services will eventually get involved. I don’t know too many people who can successfully support themselves and a child without ever setting foot into society again, so they’ll have to show up at some point.