You could use any of them, but I would probably go with North America. North America is fun because they can relate to it, because it is where they live. And as you work though history it will relate back to geography as well.
I’m doing American history for 1st grade too, and we’re just going to cover it as it comes up. This past year we did learn the continents and countries of the world, but if you haven’t yet, you could always do a brief discussion of that and then we will for example, as we are discussing Leif Erickson, we’ll make a map and trace his route, same with Columbus. We’ll look at where Jamestown was on a map, and John Smith explored Chesapeake Bay, so we’ll find that. I’m not doing an official geography thing, but for me it makes the most sense to have it just be a part of what we’re doing as we do it!