For me, and probably many artists, painting a picture is like a study in layers. I almost always start with a basic sketch, usually not very detailed, of the painting I'd like to do. I start with very basic ideas, but I allow my ideas to evolve as the illustrating process goes on.
If you look at my original basic sketch of this painting I just finished of a Barbarian Woman riding a grizzly, you see my original idea was to have the woman in a very different pose. But once I finished most of the background and added a few colors to the woman, I decided I didn't like her pose because it didn't look natural. Therefore, I colored over it and did another very loose color sketch of a woman in a different pose. In fact, I tried several poses before I finally landed on one I like.
Never be afraid to change a picture for the better, even if you've already put a lot of work into your picture. I once scrapped a children's book I had worked on for months, just because I thought the main character didn't resemble herself enough throughout the book. That was a painful decision, but one I don't regret. I knew I would never feel comfortable publishing work that I hadn't put my very best into.
Remember, your audience, no matter what age, will appreciate your hard work and attention to detail.