Hello my name is ***** ***** i will be happy to offer you some guidance on this issue.
Unfortunately for security and privacy reasons its almost impossible for a regular citizen to unlock a phone without potentially permanently damaging or wiping clean the data in that case it will be of no use for you if i understand correct. Only law enforcement agency with some very special tools can have such access. Again not the answer you were expecting but unfortunately that is reality and your best bet is to work with grand daughter to offer the access directly for her own good.
I hope this gives you some clear direction on the current situation. Let me know if you have any further questions. Please don't forget to accept the answer. Thanks and good luck.