Enman Katei no Hitozuma ga Musume no Tomodachi ni Dohamari Suru Hanashi

Yuki Amamiya is a highly regarded designer with a stable life. She has a good relationship with her husband and daughter, and they have a dog together. It’s a harmonious, picture-perfect family. She never harbored a single doubt about her family and believed this life would smoothly go on forever. …Until that day, when she met Kai. She was her daughter’s classmate — childlike in appearance, yet with words, actions, and a gaze that challenged even adults. But Yuki couldn’t ignore the occasional flashes of insecurity that were so typical for her age… “I don’t mind you touching me, Yuki, but you're getting me all excited.” “…You perverted brat.” This is the story of a happily married woman who ends up doing XX with her daughter’s friend. (Source: Dengeki Bunko, translated)