"For 5 and 6 yr olds with limited artistic skill (for now!) would you recommend coloring pictures related to stories/poems when making books about them, until their skills have improved (I am thinking of doing a sketch journal for them to practice, that doesn't have to be "perfect")?"
You could do this, and I occasionally do it with my five year old, but generally, I prefer my child to create the picture in his mind rather than rely on a black and white simple drawing to take the place of developing a vivid imagination. There's nothing worse than creating a beautiful image in your head about a tale you have just heard only to have it replaced by a simple cartoonish picture that you must color. I've done this to my children before and have heard in a disappointing voice, "Oh. That's not how I pictured him to be."
Granted, you are helping your son develop fine motor control by teaching him to stay inside the given lines, color in a particular direction and also distinguish the difference between light an heavy shading by applying varying degrees of pressure. These are valid skills and if not overemphasized, certainly helpful. I would just caution you against relying on coloring pages too much. They really do stultify the imagination. Although some children naturally love coloring and will not complain, a hurdle has been placed in front of the skill of developing a beautiful imagination. We use coloring books, but generally keep it separate from most of our read aloud tales.
Drawing can certainly be postponed if a child really dislikes it, but I've learned that the sooner he becomes comfortable with it, the easier it will be for him when older. Practice, practice, practice is so important in order to develop this skill. My five year old son draws at least once a week, but he uses a very simple drawing book that guides him in copying simple animals from shapes in just four steps. This makes him feel successful. Now he draws constantly, whereas at the beginning of the school year, he was extremely frustrated at his scribblings and was afraid to draw.
I refrain from requiring drawn narrations at this early age due to the frustration it generally creates- he simply can't get that beautiful picture in his mind down on paper. Of course, some children don't mind a bit and love to draw what they have in their mind's eye. I think that is wonderful, too.
one step at a time...