oh interesting question i think this is the one thing where shouto freaks out more loudly so to speak because not only is this a thing directly happening to his body he's also got the great example of how he was raised/spent his childhood to think of so any initial worry katsuki might also feel would have to take precedence only after he's calmed shouto down first

conversely shouto could be outwardly really calm but i think katsuki would still clock it immediately that he's trying to put on a brave face

as for the general "oh no what do i tell mom and dad" aspect of it i'm not sure honestly? bc they are very much canonically child soldiers so uh... assuming we're sticking to that sure parenthood at that age is a whole other thing but i do think that at least at an emotional level if they do decide they wanna go through with it it would be, like, a less heavy choice?

not sure i'm making sense low key wanna spin this idea like a rotisserie chicken in my head some more and see if i land on an actual fic now