Pantalettes became less common in the 1870s, but were worn by some boys into the 1890s. Pinafores Little boys in the early 19th Century still in dresses might wear pinafores. This was more common in the early half of the century, but was not unklnown even after the turn of the century. I believe it was mostly boys still in dresses that might wear a pinafore, but have little evidence to substantiate this. It was also probably more of a play garment to be worn around home than a garment to be worn for dress occasions.
Some period movies depict boys in long hair and dresses or kilt suits.
Movies of course can vary greatly in historical accuracy. It is likely, however, that those who have gone to the trouble of depicting boys in dresses and long hair, probably ercised considerable attention to detail.