Naruto might first be in denial. He would find it hard to believe that his mother would break a promise to him. But as the truth sinks in, he could become angry. He might lash out, either physically or verbally, at those around him who remind him of the broken promise. Eventually, he would probably turn this pain into motivation to prove himself despite the setback.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.