It happens to a person's life once or never. However, if you see the statistics, you will see that it is happening everyday. So, shouldn't we be discussing on this? What if next child is yours closely.
Its panic for the relative. For others, it can be a guess job. Some may try to help by being with the sad family and other may like to help by idea!!
Kidnap
Accident
Love affair
Child smuggling
Child lost way back to home
Inform police
Notify relatives and friends and ask for help
Look for child phone, if available
Sometimes, people feel it can be love affair. However, in this case, there will be below tracing in past
Child may be away repeatedly and parent don't know what child did during this period
Child may be in call for which parent doesn't know
Child expense level changes
Child may have items for which parent doesn't know when child gets it
Are you agreed? However, then parent asks how to get above info. Parent should think about this. Note that any strict act to child will not help.
If you don't know these info, then you will be harmed multiple ways. You may falsely think something as love affair which indeed could be a very genuine case of kidnap etc.