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Your wife is in the wrong, she shouldn't teach your child it's OK to be a scumbag thief. You should ask your wife if she'd want someone to hand in her phone if she lost hers.
This, and i'd say a 'reward' for your son for being honest....
Shame people don't reward willingly, but I think if someone handed in my phone i'd want to give them something for saving me having to resort out all my contacts/data/photos/number etc. Bottle of wine / Chocs/DVD depending on who it was.