What child is going to choose to ignore millions of dollars? But true, that is an option I suppose.

Agreed, it's highly unlikely. But they have a choice, it's not forced by anyone. If they do nothing (maybe their parents never tell them, or they don't read news, or facial recognition never informs them of similarity), there's no inheritance. If they take consensual action to make a claim via DNA paternity test, the inheritance can be claimed.