As Dindrane pointed out, the caregiver has responsibility for those behaviors the caregiver can reasonably influence. Putting away a small portable expenisve item would be somethign easily influenced to prevent the distructive action. I see leaving an iPad on a coffee table where young children can access it without appropriate monitoring as very careless.
I did say that the caregiver has responsibility for behaviors they can reasonably influence, but since Herman was being cared for in a non-usual environment, I don't think "putting away every single valuable/breakable/expensive/potentially dangerous thing in a place where Herman can't get to it" is something most people would be able to reasonably do.
If the nanny left her iPad on the coffee table at Herman's
house, where his parents presumably live in a state of "keep the valuables/things we care about/expensive/potentially dangerous items far away from Herman," I'd agree more with the idea that her negligence was partially to blame for its destruction. But I don't think that is equally true at the nanny's
house, especially if she often keeps her iPad on the coffee table when she's not using it.
I don't live with children, and I don't have pets. As a result, I have things that could be dangerous to a 4 year old lying around. I have expensive things I care about out where there are easily accessible. My computer pretty much lives on my coffee table, because I mostly use it when I'm on the couch. Even with, say, a week's notice, I might not be able to put all of those types of things in an inaccessible spot. For one thing, I don't have a lot of extra room. For another thing, I'm not necessarily going to realize in advance all the things that could be problematic so that I can move them.
In the case of the iPad on the coffee table, if the coffee table is similar in color to the iPad, or it got obscured by something, or it's even just where the nanny habitually sets it down, I can see overlooking it. I don't always actively notice that my laptop is on the coffee table, because my brain has filed that away as "object that is in the right spot." It would take some effort on my part to reject that previous idea and form a new one about where the laptop should go, such that I would notice it on a quick scan of the room to see what needs to be put elsewhere before a child like Herman visits.