Children sleep more soundly than adults and I watched a video that clearly showed this. The parents were in the video. They set off a fire alarm. The kids did not stir.
There are fire alarms that record the mom's or dad's voice telling them there is a fire, get out now, wake up, get out now.. type message. The same parents recorded a message into the fire alarm and then the video shows how the kids woke up instantly when the alarm went off because they recognized their parents voices and followed the instructions.