Hey @darknova,
This is technically a “General Website Question” and not MainWP related, however, I’m going to answer it anyways because I think MainWP can help the situation.
With that said, as you stated you know what a 404 means, for others that may not know, that error is thrown when a URL is not reachable and or not found. If you visit that link ( which I am going to take a wild guess isn’t one of yours or your clients ) you will see that the SSL is broken hence the reasoning for the 404.
It’s great that you scaned for viruses, however, I think this is a comment spam thing. Someone has injected that link into a comment on that site somewhere; In turn causing you to recieve those 404 errors when your traffic hits that link.
My suggestion is to do some thorough comment maintenance on your Child Site.
Here is some information on that topic: