Tensei Arasaa Joshi no Isekaikatsu

An ordinary woman in her 30s wakes up to find herself reincarnated as Magnolia (3 years old), the daughter of a prestigious marquis! What she thought would be a happy life in another world awaits her, but what awaits her is mistreatment from her family...and an unwanted political marriage!

(Source: Tonari no Young Jump, translated)