I'm not the parent, but I'm an American who has been living abroad for ten years.

The banks know where the commercial mailing/forwarding services are. They somehow keep a database of them. They're fine with you using a mail forwarding service as a mailing address, but they will want to see a residential address where you live. If you can't show a residential address in the United States, banks and brokers can and will freeze or close your account.

I've learned my lesson. Pro tip: never tell the banks that you're living abroad. You're always taking an extended vacation. And always have a trusted friend or family member whose residential address you can use in the United States even if all your physical mail gets sent to your mail forwarding service.