I have used a namechange here as I don't want to post this under my regular name.
My DS used my bank card to buy himself online Nintendo DS games, the bugger. I think he was 10. He strenuously denied it, in floods of outraged tears "how could I suspect him of doing such a thing, how could I call him a liar." In the end I found the games in question on his DS with the exact times they had been downloaded, which matched the times the bank had given me. Presented with the evidence, he finally admitted it. He was VERY sorry. Yeah right, sorry he was caught more like.
There were two transactions, one of £9, one of £18. Having proved the £18 was him I said I would let him off paying the £9 back if he stopped lying and admitted it without me having to find the proof first. He admitted it. I bitterly regret not making him pay that £9.
His punishment was that he had to pay the £18 back, and lost his DS for a period of time. Can't remember how long. I now really don't believe him about stuff and trust has been lost. Looking back I don't feel the punishment was harsh enough. I was heartbroken, frankly.
He is a very well behaved child and if you knew him you would be astonished. At the same age I used to take money out of my Mum's purse for sweets. I did this over a period of about 6 months, 20p, 30p a time until suddenly one day I was stricken with remorse, cried myself to sleep with the guilt and have never stolen anything again in the rest of my life.
When the subject of the stolen money is brought up - and I can't help myself, I do bring it up, usually if he is saying "but why don't you believe me" about something - he seems genuinely sorry. Hmmmph.
I think kids just do things without thinking. I hope to God my DS doesn't try anything of the sort again. I know that there is absolutely NO benefit of the doubt given to him any more.
Hope this makes you feel a bit better, and a bit less alone, OP.