Mitame Youjo na Akuyaku Reijou wa, Koori no Koutaishi (Tokidoki Kawaii) no Ude no Naka.

For the sake of her kingdom, Juliana, a duke's daughter, becomes the fiancée of Folkhart, the notoriously icy Crown Prince of the neighboring empire. With magic her appearance is de-aged into that of Princes Liane, a 4-year-old girl...

According to Enzo, her valet who claims to have arrived from another world, this world is actually an "otome game," and Juliana is the villainess doomed to total ruin. Oh no... at this rate, if the ruthless Folkhart uncovers the truth, both she and her nation will be wiped out!

"Buh I weefyooz to gib up!"

(Source: Kadokawa, translated)