Did your grand daughter replace your account with her own account (a factory reset)?
Or did she simply add her own account to your tablet and now for some reason it will not accept your own password?
With the update to 4.2.2 it is possible to add multiple accounts to a single Android device. Perhaps you encountered a bug? Ask your grand daughter to unlock the device and see if your account is still there. I never added a second account to my 2012 Nexus 7 so I am not familiar with the steps.