It may not be exactly relevant, but the word حكاية can have a more "mythical" or "fable" connotation. For example, when you talk about telling a fairy tale in Arabic, or when children want to talk about stories, they would rather say حكاية. Also قصة has a kind of formal feel to it. They can definitely be used interchangeably without any problem, it's all about those subtle differences. In this particular sentence I feel like it is used for that "bedtime story" connotation, to make it seem less formal and more familiar, is that makes sense. Hope this helped.