Depends on where it is. There are places I wouldn't send a 9 year old boy to the men's room alone. We had a public park near where I went to college that had to be shut down because the men's room (and then eventually even out in the open) became a hook up spot for gay anonymous sex. No way would I let my kid go into a bathroom alone if that is the situation. Not just because I'd worry about what would happen to them, but of what they might see if they walk in there. Although, sometimes the women's room isn't much better. At the subway station in NY on my last visit, the cops had to come in and make the troons get out because they were holding up the line while they were in there either shooting up or sucking each other's lady dicks.Who would bring a nine-year-old boy into the bathroom with them? Maybe if they're profoundly disabled?
A bathroom in a decent area, though, I'd let a 9 year old go in alone.