This is a very important question and from the response it shows that parents have developed all ways to make sure they track their children in their absence.
There are cheaper tools available as @Mercedes07 as asked.
There is the Buddy Watch by EveryNanny, this tool allows you to know on real time when your child is not in a good state. It alerts you when the child's heart beat is racing and this is most likely when the child is crying or playing hysterically. Even if you call immediately to find out to confirm the state of the child, you are 95% sure of what is happening and no one could give you false information.
Also too, the Buddy Watch beeps when it is removed. So it's a well round tracking tool.
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