How do I reuse a old iPhone from a family member that has passed away? :(

How do I reuse a old iPhone from a family member that has passed away? :( ...we were going to through some of grandpa's old stuff and came across his iPhone that we had given to him a few years ago ...but sadly he was the only one who knew what he had as a password ...the phone still seems like it is usable ..well it does hold a charge after being plugged in for a day ....but it only goes to the screen that says Activation Lock ...which again was grandpa's ID and password ..Does anyone know how to undo or remove this? ...We don't know what if he had any photos on it ...I'm sure it did ...but apart from that we have the memories that we have ...We are wanting to give his phone to our daughter who is just turning 6 and it would be nice for her use something that her grandfather used before ...we already put to use his old work office desk and chair in our daughters room which she uses to do her homework ...it's actually the desk which we found the iPhone and some actual photos in ...any help??