I'm trying to find a fic I'd read a couple months ago where Steve is de-aged and Bucky takes care of him. Bucky is still not really himself at the time and not everyone really trusts him but he still takes care of Steve. I can't find the fic anymore and I only remember a few things that happened in it:
— after Steve was de-aged, Sam(?) says that he thinks Steve is a couple months old or a year old or something but Bucky speaks up, which he doesn't usually do, and says that he's three
— in the fic bucky calls steve sunshine (or some other pet name but pretty sure it was sunshine) but in russian.
— Bucky gets a new arm towards the end of the fic but at first it's really sensitive and Steve startles him and Bucky accidentally hits him across the room, he thinks he kills Steve and get so upset he runs away
— Steve sang a kids song (wheels on the bus i think) a bit while de-aged and the avengers record him and try to spread the video so Bucky will see it and realise he didn't kill Steve and come back
I think it was on AO3, might have been Steve/Bucky but was more likely Steve & Bucky