Hi Bharadwaj,
You need to consider some points. Please look at the https://www.nxp.com/docs/en/user-guide/JN-UG-3113.pdf
B.2 Sending Data to Sleeping End Devices
The stack will handle this request and answer back setting the frame pending flag.

The device won't enter to sleep mode and it will wait for the data that the parent wants to send.
For better information please look at the next chapter B.2.1 Acknowledged Data Transmission to Sleeping End Device, you have 7 seconds after the initial transmission for the SED ack. It is important that your end devices should be synchronized with the wake-up time of your device.
However, after the join request, the parent receives the timeout. You could take this time as a reference to your device.
Regards,
Mario