You mean a Nigerian traveler, who is coming here for just 2 weeks to scam his lonely Instagram followers, can open an account with Interactive Brokers with just his passport number? Geez, I hope not. I still think only resident aliens who are living here are allowed to open an account. If anyone knows, let me know.
Everyday we open foreign accounts for those that are citizens of many counties. Instead of a W9, they fill out a W8BEN. The only withholding is what the IRS calls NRA-on Dividend Income only and varies based on treaties with the USA. The base rate is 30%. The customer is responsible for reporting their gains and losses with their country they pay taxes in.
Just trying to be helpful here because many others do the same thing. First, you didn't actually "cite a source" or what's technically known as an "authority." Unfortunately, the general information on the IRS website, such as the url you provided doesn't actually "mean anything" (same is largely true when a rank and file IRS staff member gives advice, it doesn't "bind" the agency, although it can sometimes be used persuasively to reduce penalties and or interest). Second and perhaps MORE important, being able to cite a regulation doesn't mean it applies as the final rule to any given taxpayer. In other words, under one IRS code a certain set of facts may equal no tax obligation, while another IRS code may say something very different, as well as 'overrule' the first code.
The only way you will receive an answer to this question is to obtain an answer from a qualified Tax attorney, CPA, or EA (in that order of preference). The biggest problem I see with your question is in the first sentence, namely asking the internet won't hurt either as I have many clients that found out the hard way how discussing a matter without attorney-client privilege was harmful. Regardless of the jurisdiction, get the answers to all the questions, including the ones you haven't even thought of, answered ahead of time.
It's very likely that your country has a tax treaty with the USA, so that's something you'll have to figure out. I would contact an accountant or tax attorney in both the USA and in the country you are living in. What i am wondering about, how did you get a withdrawal straight from your brokerage account to your childs bank account? I was under the impression you could only withdraw to accounts in your own name and sometimes even only to the account you made a deposit with.