Loki was turned into a kid (i dont remember how) and thor runs away with him to a beach side house, and enrolls him in school, and just generally tries to take care of him. I think thor was trying to find a way to turn him back, while also struggling with wanting things to stay the way they were. At the end loki returns to being an adult, and i think they kind of reconcile a little bit?
It's been a very long time, i hope these details are enough tho, i really want to bookmark it and leave a comment like i should have when i read it.
ETA: turns out it was actually thor who was turned into a kid, and loki who took care of him, whoops!