Who was the smirking kid...

...who opened the door for the nervous landlord (at the Genco office) in GF2? Young Tom?

It was Genco wasn't it..

Tom would have still been a baby, and the Corleones didn't know him yet. He and Sonny met when they were about 10, if I'm not mistaken.

Not Genco- this kid was late teens-early 20's, wearing (if memory serves) a soft cloth cap. When the landlord fumbled with the knob trying to beat a hasty departure this kid calmly opened the the door, with the afore mentioned smirk.

I can't swear to it (my memory is fading) but I'm pretty sure that was Genco who opened the door for Sr. Roberto.

Of course it was Genco. Don't you remember the scene when Genco took Vito to the opera to see Genco's girlfriend? It's the same guy? Would Tom have been as old as Vito? Don't you remember the scene outside their apartment after Vito kills Fanucci and all of their kids are gathered around including Sonny who would have been about ten or so?

It was Genco, there were other people doing something outside, but it was definitely Genco who opened the door.

I thought it was, i nearly had to watch the scene again just too double check..

Well, you all are right. Consarn my senior memory. I just slapped the dvd in and...of course, it's Genco. Just for the fun of it I went here to triple check: http://www.imsdb.com/scripts/Godfather-Part-II.html
If you haven't checked it out, it's kinda fun. the movie follows this script pretty close, but parts are different.

Michael is a newborn in that scene. Is there really 10 years between Sonny and Michael?? Sonny might be closer to 10 when they go to Italy (when he gives the older woman the Statue of Liberty), but that's several years later.

In the movie, Sonny was about 4 or 5 when Fanucci was killed.

Well, some of the novel bleeds over. There, Sonny follows Vito across the roofs when he's about to kill Fanucci. I can't see a kid much younger than about 10 doing that.

You're mixing apples and oranges.

Sonny never saw Vito kill Fanucci in the movie. Yes, in the novel, Sonny was born in 1910 (he's described as being 35 years old at Connie's wedding in 1945). In the movie, Sonny was born in 1916, so there's six years difference between Sonny's age in the book and the movie.