This one is less clear cut to me than the laptop, glass of water, cat one.
On one hand, Annie is doing the family a huge favour, getting Herman away from the house during the construction.
On the other hand, she is well aquainted with his destructive behavior and left out an expensive item within easy reach.
As the parent, it might depend on how good a nanny Annie is. If she is fantastic with Herman and they really love her, they should pay for the iPad in the interests of good will.
Otherwise, as the parent, I would offer half the replacement cost. Because the nanny was doing me a huge favour but ultimately, Herman was my child.