You're a LIAR I DON'T BELIEVE YOU
This is the your first time writing a story?! HOW DARE YOU omg I'm so ashamed of myself now HAHAHHA (I'm joking about the first sentence but seriously, just one word; Wow)

Giving constructive criticism... sad to say I don't have any because really, I don't find anything wrong or need to be given advice about this story. But I can give some simple opinions about this story, though it isn't some advice or anything for you to gain or learn from so you can take it as a pinch of salt.

The difference between a movie and story-writing isn't only being that one you see and one you read. There's lots of talking involved for the former for it to work out and be called a "movie", and that's where the beauty of a story-writing comes about; you don't really need dialogue to portray a story, but instead focus on events and emotions to deliver the plot. And that's exactly what you did.
I like your style of writing. It's easy to read and isn't filled with tons of fancy, impressive and unnecessary words that an author may use to impress the readers. Straightforward in a sense that you get the imagination into the reader's head and make them understand the feelings of the characters. And yes, that's exactly what I, as a reader, wants.
TLDR: You know what type of words to use to correctly execute your story

And despite this being a one-shot, it is legitimately is a one-shot. There's a beginning, a plot, and a ending. I'm at fault sometimes when I do go one roundabout in my one-shots or multi-chapter fics, and ended up creating many useless filler-subplot that isn't necessary for the story, but well, you didn't. You inject good ideas/plot, extract good memories from canon-setting of the anime, draw them out into realism and tie this story with one beautiful gigantic ribbon. I do described Kaito and Aoko's hair as wild or messy, but I've never really considered the genetics and how their future children's hair would be, which is interesting and fairly refreshing to read as a good and light-hearted one-shot. And I like those little extra characters/moments you add into your story, like how Jirokichi was in rage because he was at the back of the newspaper hahaha! Not forgetting about other side (yet also impactful) characters is important too! (Like Akako, Keiko etc)

Overall, I like your writing and I love those little moments and events of Kaito and Aoko's life as their age grew! It's really refreshing! And your story made me miss Toichi more that I usually do :'(
And just a little (probably pointless) encouragement, Don't chicken out! (I'm glad you didn't and that you've posted this story in the end!) Okay, it does takes a lot of confidence to post your story up, and the amount of recognition it gets does impact a lot on whether you decide to continue writing or not, but believe me, as long as you keep writing, one reader from the future will thank you in their heart for it, even if they are too lazy to tell or something. Okay, enough of my irrelevant philosophical "advice", and for now, I'll wish you the best in writing :)

This is amazing! I am giving you a round of applause for your beautiful work with Aoko and Kaito. I couldn't find anything wrong with it at all! I loved every aspect of this (especially the comparing of Kaito's hair and porcupines) and would love to read more from you! It is absolutely awe-inspiring that this is your first fanfic.

I am afraid I am terrible with constructive criticism, and don't have any for this one-shot.

Thank you for your stunning work! Keep it up! (When you have time, of course)

This is funny and awesome I like the twist of plot tho.

This story was great!

wow...! that was cool... i think you have a really nice writing style... even though its your first time... it is really good for a beginner ... looking forward for more of your work...what was on the last tape by the way...? curious to know...

Very nice! I enjoyed this story, and I wish you the best of luck on your writing!

your style is simple and funny, please write more so we can enjoy it longer :D nice job :)

Ohhhh it was greattttttt. You write amazingly.

Oh yeah btw I would love to know what exactly was on that aoko's last tape. You made me sooo curious:-P

Oh wow I can't believe this is your first time writing a fiction. At the end of story when I was thinking yay I found another fantastic writer of kaito aoko stories, I read that this is your first time. I seriously couldn't believe it. Honestly, this story was amazing. Your writing style and ofcourse the story and characterization, it was all perfect. Do write more..we would love to read them:-)

I absolutely loved it! To think someone would take such a tiny detail and make it into a story forced me to click on this fic! And sweetie, you don't have to be so underconfident. You've worded it marvellously, and I couldn't find even one mistaketrust my eyes). Very very impressed!