Okay, read Chapter 7 on managing end devices:
End Device Poll Timeouts
To better support mobile end devices (end devices that can move around in a network), parent router and
coordinator devices have a poll timeout for each end device child. If an end device does not send a poll request
to its parent within the poll timeout, the parent will remove the end device from its child table. This allows the
child table on a router or coordinator to better accommodate mobile end devices in the network.
and from page 91:
The child poll timeout is settable with the SP and SN commands. SP and SN should be set such that SP * SN
matches the longest expected sleep time of any end devices in the network. The actual timeout is calculated as
(3 * SP * SN), with a minimum of 5 seconds. For networks consisting of pin sleep end devices, the SP and SN
values on the coordinator and routers should be set such that SP * SN matches the longest expected sleep
period of any pin sleep device. The 3 multiplier ensures the end device will not be removed unless 3 sleep cycles
pass without receiving a poll request. The poll timeout is settable up to a couple of months.