Is it best to just let them draw what they want, on their own terms, without really interfering? I considered trying to give them some guidance, actually 'teaching' them to draw things a little better, but I figured that'd be a little pushy, considering their age.
Is there anything you can suggest to foster this nascent skill, or is it best just to continue providing the pads of paper and the colorred pencils (we go through about 10 pads/week), and let them figure it all out?
They're only four, so I think it's best to let them be for now . If you start putting pressure on them to draw in a certain way it might become less fun for them and more of a chore.
If you want, every once in awhile when you see them drawing you can try and give some tips, but make sure your compliments outweigh them, because they may take suggestions as criticism, and you don't want to discourage them. I wouldn't suggest overdoing it, nor do I think you should try and have "lessons" with them at such an early age.
And you're right, letting them draw and draw and draw will help them develop and improve.
As Adhoc said, you could try giving them new art supplies, different mediums to work with, and see what they prefer. You can also provide them with some books with reference material if you don't already have some, with the dinosaurs and bugs in it that they would like to draw.
Also, I see that you're in NH. I'm sort of a NH-phile, myself - lived there most of my life and have explored most areas of the state. I've lived in Nashua, Litchfield, Manchester, Milford, Barrington, Somersworth, Dover, Hampton Beach, and Francestown!
Ah, I've only lived in Nashua, thus far. I haven't been around up north really, so I don't know those places too well. Nashua is a pretty nice place. Why did you move?