The nanny's job is to look after the child. Part of that job is to keep the child from damaging objects. She is being paidto have rresponsibility for the child.
I completely agree with the above.
The nanny knew the child had a history of somewhat destructive behavior. It's not the first time nanny watched the child. Perhaps the first time in nanny's home, but nanny should know the history by now and be prepared for the child's possible actions even in her own home.
My question to nanny would be "Where were you when he grabbed your ipad?" Even if the answer was "I just turned my back for a minute" I still think the nanny was negligent by leaving the ipad within easy access.
If nanny answered "He grabbed it out of my hand, ran to the stairs and just threw it" Eh. I still might answer "What were you doing on the ipad when you should have been paying attention to my child?"
I think the onus is on nanny . . . for not being diligent.
However, my answer will probably change if this was only a first/second even (possibly?) third time of the nanny watching the child where she really didn't know the child at all. But it doesn't sound like this is the case.