How to know which Apple ID is used to lock iPad if the display is dead?
Unfortunately, it is likely that your nephew temporarily used someone else's Apple ID in order to gain access to purchases that the other user had made.  My son did this on his device, against my rules.  If this happened, then that other person may have initiated the lock.  Since you have an invoice, proving ownership and the device is only a month old, I would encourage you to use the 1-year AppleCare that came with your device.  Here is the AppleCare contact page for Saudi Arabia.

Please let me know if this information is helpful for you.  Did you by chance register your device with Apple, when you made the purchase?  If so, then Apple will have a record of that action, as well.

Cheers,
Byron