At the same age, with almost identical circumstances, my brother did this.
Except he didn't stop at the money in the account and created an unauthorised overdraft. Which incurred charges.
He was gambling it away in arcades.
Mum didn't tell the bank. Went through a very hard 18 months to pay back the loss. He wept. Promised it would never happen again. She got him help for a gambling addiction. I sat with a deep seated, but persistant feeling that behind the tears there was a growing realisation that family was an easy mark.
By the time he was in his 30s he had taken my mother's house from under her. Abandoned his pregnant wife and their children. Declared bankrupt with a debt containing more zeros than my open jaw can cope with. And yes, gambling featured again. But there was something else. With the tears, and the lies, and the promising. I saw it, where it had been for years. The sly understanding that family is an easy mark.
I can't be near him. Haven't seen or spoken to him for a decade and half. He can't, or won't change. And the urge to whack him over the head with something very heavy is too strong to risk any sort of contact.
If I had a time machine I would call the bank and the police myself. Hurl the windows open and let sunlight bleach the secret keeping out of what used to be my family.
I can't tell you (without a convenient parallel universe) if he would have turned out differently. Perhaps the shock would have made thieving too risky a prospect. Maybe not. But at least he might have thought twice about his own flesh and blood being his mark of choice. And as a family maybe we wouldn't lie in quite so many tatters where biological bonds used to be.
It can get old fast, watching somebody make excuses for the inexcusable, not being able to cope with the economic/emotional aftermath and needing support, again and again and again. It is no fun being labled "the mean one" because you are left with the role of having to argue for tougher reactions, to protect the person bewailing yet another fleecing.
There is nothing my son can do that will make me turn my back on him. But were he to steal from our bank account as a minor, I would report him. To the bank and to the police. Because I love him. And IME the alternative carries just as many risks, perhaps more risks, of a lousy long term outcome for the kid and everybody around him.
I wish you luck, strength and fortitude. I wouldn't wish this on my worst enemy. And I sincerely hope it turns out to be a blip, rather than an indication of the future, for your family.