I'm a native American English speaker, and I would just say "say 'hello.'" It would be grammatically correct to say something like "Say hello to this gentleman," but if you don't know the name, just "say hello" seems like the standard. If you're wanting the child to say "hello, sir/ma'am" (which wouldn't be standard at all for children in the US these days), I think you'd have to prompt that at another time, with "when we say hello, we say 'hello, sir' or 'hello, ma'am,'" or something like that.
37, hoping to have a new member of the family in 2016, to join my queer clan: Me
, Things 1&2
, my long-distance KD/cheerleader (the guy who's been telling everyone what a great mom I'm going to be)
, and the rest of the superheroes and sidekicks