Yo guys, I found some quick but cute male and female AR in chapter 6 of the manga version of
Gakuen K.
I have no idea about the context (The only thing I know its that it's a comedy), but when a dude gets kneed in the gut by a teacher, his friends Kuro (guy) and Neko (girl) rush in to help. Then some black thing I couldn't identify comes in and transforms Kuro and Neko into a child and a baby, respectively. They're not shown returning to normal at the end of the chapter, but considering it's only the sixth chapter of apparently four volumes, it's safe to say they did return offscreen.
There's no process, since it's a gag manga; the only things we see is the characters gasping as they glow, then it cuts away to another character finding them regressed. Still, it's adorable and leaves them in oversized clothes.
https://comic.pixiv.net/viewer/stories/5934
(Sorry for only being able to post one rather small attachment; the only link I found didn't allow me to save the image directly so I had to make a screenshot)