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.

the real difference between (Hikay حكاية, and Kissa قصة) is that: Hikaya shorter than Kissa ... قصة a few lines or one page .... حكاية can be like a long story ...

حكاية = tale قصة = story

I think that قصة is longer than حكاية so in your sentence that his حكاية ( as part of story) will change our قصة ( the whole story) I hope you can understand that

thank you!