The protagonist is Paul, the little boy.  He is striving to, through the riding of his rocking-horse, win the affections of his cold and distant mother.  We can't help but relate to that plight, and our hearts go out to him in his quest.  The antagonist would have to be his mother, her aloof nature, and her incessant quest for more money that, in the end, takes the life of her son.  She is painted as shallow and vapid, unwilling to work hard or manage her money well, and as uncaring and cold.  She is the "bad guy."  Another possibility for the antagonist is greed itself; it is the evil demon that drives this family to tragedy and ruin.

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