May one dress their children in clothing of the opposite gender?
Children or all ages [even below Chinuch and even a baby] may not wear clothing of the opposite gender unless there is no other clothing available and they need to wear it to protect from the cold or the sun.
 Igros Moshe 4/62-3
 See Minchas Yitzchak ibid
The reason: As it is forbidden for an adult to dress even a child with men’s clothing as it is forbidden to feed a prohibition to a child of any age. [Minchas Yitzchak ibid, based on Admur 343/2]