This is just extraordinary. I adore the plot! It's so unique and clever. I generally don't go for de-aging fics, and time travel can make my head hurt, but this isn't quite like either of those genres. Congrats on coming up with such a fresh and unique idea!! The level of domesticity for both Snape and Filch was also just PERFECT. They clearly come to care for the kid, but neither of them ever hit my OOC or fluff button at all -- not even close. They were both just fantastic. Adult Snape especially just reacted to the kid absolutely perfectly, right from his very first words to him. :))) And good grief, IDK how you managed to do a kid's POV so perfectly and non-annoyingly in those segments, but you did. He was charming and sweet and naive and hilarious, and I loved his observations of Snape and Filch. And Filch, GOD, so rough and sexy and funny in all the right ways.
I had no idea how you were going to end this, and I admit I was really surprised at the direction it took. Maybe I've been reading too much darker fare lately, but I totally thought (as much as I could guess at all, which wasn't much!) that Snape would have to kill himself to allow the boy to grow up in his place (although in that case, I couldn't figure out how he wouldn't remember it happening). But I'm kind of glad you didn't go into mindfuck territory or darkness or tragedy. Somehow, you pulled off this brilliant happy ending! I still have some time-travel questions, but then... so does Snape, right? So, I'm happy to accept it all on the level that he does himself, and wander off into the Canadian sunset with them. :))) Brilliant storytelling.