Who is at fault if a kid loses your cell phone?

My cousin and her husband were at McDonald's. Her husband instructed their kid (a 5 year-old girl) to take care of his cell phone and he'll be right back. He got back in 2 minutes or a bit less and she already lost it.

He thought their daughter could follow easy instructions. He had called his phone call, request the number to be blocked and is in the process of buying a new one. Who is at fault here? Would you blame the kid or not?

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When he asked where his cell is, their daughter replied ''I don't know... umm over there''. He was back fast and she already lost his cell.
Who is at fault if a kid loses your cell phone?
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