I use EmberZNet 5.6.0 and use host-NCP communication.
As per the link (http://community.silabs.com/t5/Mesh-Knowledge-Base/How-long-can-my-end-device-go-without-polling-its-parent-What/ta-p/113349), the end device will removed from child table if the end device doesn't poll its parent in about 5 minutes.
#define EMBER_END_DEVICE_POLL_TIMEOUT 5
#define EMBER_END_DEVICE_POLL_TIMEOUT_SHIFT 6
In my case, the coordinator is the parent of an end device. I unplug the battery of the end device, but after enough long time, the end device is still in the child table.
>plugin stack-diagnostics child-table
# type id eui64
0: Sleepy 0xE9EB (>)000D6F000B723223
1 of 6 entries used.
EmberZNet is somewhat obsolete. I suggest you to use EMberZNet 5.10 to have the test again.