The Bnha universe's reaction would be mixed. Those who are into complex stories, like Todoroki, might study the relationships in Hamilton fanfiction. But others, like Mineta, might not be interested at all because it doesn't have much to do with his usual interests. Overall, it would depend on the individual characters' personalities and interests.
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.
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.