Children often get tired or bored, and quit at just the right time to leave a fresh "take" on what they see, or are trying to express. Having raised two kids I can state that children are simply lucky in knowing when to quit. More often then not a drawing would turn into mush from overworking and revision. Once or twice a year (after dozens of recycling bins filled with "keep-em-busy" sheets) a beautiful scribble turns up.
The inexperienced artist usually feels a need to work to some sort of perfection. This is just as well since the novice needs the experience of working more then the she needs the perfect masterpiece. The skills will come with experience; and mastery will lead to masterpieces. Just as the novelist needs to learn spelling, grammar, and sentence structure before telling a story, the artist needs to feel natural and comfortable with his tools before expecting perfection.
Unfortunately, the modern world seems to encourage an impatience in people. If I can photograph my house, or model and render a design in no time, why can't I create perfect beauty instantaneously. Life is short: meaning that we expect to experience everything in the world right now! I guess I'm more a stick-in-the-mud then I thought. In reality life is longer and better then at any time 'til now.
The mature artist, having mastered the tools and techniques, can return to the visceral reaction of a child. Although composition and proportion can be taught, beauty tends to happen when you least expect it. Letting your skills race along, while your mind's eye stays awake to the possibilities that are developing, seems the best way to get to the right place.