The password should be stored in your niece's laptop, or any other computer which has connected to that router.
If that computer has Windows 7, there is a built-in function to display the password. Go to:
Start > Control Panel > Network & sharing > Manage wireless
That will being up a list of routers. Find the correct one, right-click and select "Properties".
will bring up an info box for that router. Select the "Security" tab,
The security key (password) will be hidden behind asterisks. Place a
check in the box for "Show characters". That will display the password
the computer which has connected to the router has an older operating
system (Vista or Windows XP), then you may not have that built-in
password display feature. But you may still be able retrieve the
password by installing some specialized software.
Here are two suggested free software programs to display wireless passwords:
WirelessKeyView by Nirsoft:
Advanced Password Recovery by Josh Cell Software: