Koushaku-ke no Aisare Nise Youjo

The "Magic Tower" develops magical tools and potions. Sophia, the daughter of a viscount, finally gets her dream job at the Magic Tower, but what awaits her is a brutal work environment and constant constant pressure to die from overwork. One day, Sophia learns that she has been betrayed by the Tower's owner, whom she trusted. In despair, she decides to escape the Tower. She develops a potion that allows her to take the form of a child and blends in with the children at an orphanage to escape the Tower. There, she catches the eye of a duke who is looking for an adopted daughter. The duke's brother and father dote on Sophia, and she tries to enjoy her happy life as a "beloved little girl." However, a boy named Adam, the same age as Sophia, appears before her in his little girl form. It seems he has a secret...

(Source: Natalie, translated)