This style of dress was common for boys into the 20th century.
Younger boys would often wear tunic suits which featured jackets fitted to the waist that flared out to a full knee length skirt worn over ankle length trousers and a collared shirt. Boys during the 1840s wore long trousers that were often pleated and may have had stirrups, especially early in the decade.
Waists became more defined and returned to natural waist height for girls in the 1930s.
Under their skirt they wore wide ankle length pantalettes that were visible under the skirt.
Girls’ hair was often worn short and parted in the center. Similar to adult women’s fashion, girls skirts became more full and tiered.
Unfortunately, this makes dating a photo of just a baby very difficult based only on clothing.
Until around WWI, the length of a girl’s skirt was an indicator of her age.
Skirts in the late 1860s and early 1870s became flatter in the front and most of the fullness was pushed to the back. Things pretty much stayed the same for boys in this decade with the addition Faunteroy inspired by the book Little Lord Fauntleroy by Frances Hodgson Burnett. For girls from 1883-1889, more fullness returned to the back of the skirt echoing the return of the bustle in adults. Waistlines began to rise again towards the end of the decade.