It'll never be too playful, at any age, to wear a dress with a kiddish print.
Summer allows (almost) everything. But if you hesitate a bit, what about
adding a choker or chunky shoes to spice it up with a sort of boldness?
dress: Reformation(here)bustier: Forte_Forte(similar here)choker: asos(old)bag: Givenchy(old)shoes: Salvatore Ferragamo(here)