What is the ruling on dressing a younger brother (one year old) in girls' clothes for fun?

Scholars differed concerning the ruling on dressing boys in what is forbidden for men: The Hanafis and Hanbalis held it to be forbidden. The Malikis considered it disliked (makruh). The Shafi'is permitted it, according to the more authoritative opinion.

Those who held it to be forbidden cited the generality of the hadith: "The wearing of silk is forbidden to the males of my Ummah and permitted to their females," and the actions of the Companions. Those who permitted it argued that a boy is not held accountable (ghair mukallaf) and is an object of adornment.

As for dressing a girl in boys' clothes or vice-versa, it is not permissible due to the forbidden imitation involved.