The mom could be shocked at first but then gently explain to the son that it's a private and natural but not something to be begged for. For instance, she might say that farts are a normal body function but not something we should play around with in this way. She could also use this as an opportunity to teach the son about good manners.
Mom should first assess if it was an accident or if there's an underlying issue. If it's an accident, she can use it as a teaching moment. For example, she can say 'next time we need to go to the toilet'. She should also clean the child and herself up calmly and not make the child feel ashamed.
In case they were involved in something dangerous, like playing with matches, the mom has to be firm. She should explain the serious consequences of their actions, not just scold them. For example, she can show them pictures or videos of fire accidents caused by improper use of matches. And then, she should set strict rules to prevent such dangerous behavior in the future. At the same time, she should also encourage them to be more careful and think about safety all the time.
In a Giyuu x Shinobu fanfic, the setting and plot can vary widely. Some might be set during the main events of the Demon Slayer universe, with them facing demons together. Others could be more of an alternate universe story, where they meet under different circumstances, like in a modern - day high school AU. The fanfic could also delve into their backstories and how those experiences shape their relationship. For instance, Shinobu's past with her sister could play a role in how she opens up to Giyuu, and vice versa for Giyuu's own traumas.
I would recommend 'Whispers of the Heart' for Giyuu x Shinobu fans. This fanfic focuses on their unspoken understanding. It's full of small moments that build up their relationship, like when Shinobu leaves a small flower on Giyuu's doorstep as a sign of her care. Also, 'Shadows and Sunlight' is worth checking out. It has a great plot that involves them having to go on a secret mission together and how that journey changes their relationship from colleagues to something more intimate.