A few specific suggestions
When you come to the writing, remember that your story is for a child, grown-up or not grown-up, and that you must therefore make the language simple and vivid. Use a good many crude similes and metaphores. Be concrete in comparisons about size, shape, color, garb, and the like. Though you select your hero with care, you need make no fine distinctions of character, since broad strokes will be most effective. Endow your personages, both the hero and his enemy, with a few mannerisms and let them display these often. Get quickly into the action of the story and keep things lively to the end.
Working definition
Here is the working definition: A nursery saga is a narrative of imaginary events wherein is celebrated a hero of a more-or-less humble origin, a child's hero, who, by his own wit and energy, together with the possession of a charm, is enabled to do stupendous deeds, which bring to him material happiness.
Princess Helena the Fair
We say that we are wise folks, but our people dispute the fact, saying, "No, no, we were wiser than you are." But shaskas tell us that before our grandfathers had learned anything, before their grandfathers were born——
There lived in a certain land an old man of this kind who instructed his three sons in reading and writing and all book learning. Then he said to them, "Now, my children, when I die, mind you, come and read prayers over my grave."
"Very good, father, very good," they replied.
The two elder brothers were such fine strapping fellows, so tall and stout! But as for the youngest one, Ivan, he was like a half-grown lad or a half-fledged duckling, terribly inferior to the others. Well, their old father died. At that very time there came tidings from the king that his daughter, the Princess Helena the Fair, had ordered a shrine to be built for her with twelve columns, with twelve rows of beams. In that shrine she was sitting upon a high throne and awaiting her bridegroom, the bold young youth who with a single bound of his swift steed should reach high enough to kiss her on the lips. A stir ran through the whole youth of the nation. They took to licking their lips and scratching their heads, and wondering to whose share so great an honor would fall.
"Brothers," said: Vanyusha (Ivan), "our father is dead; which of us is to read prayers over his grave?"