In 1915, World War I had already had an impact. Cloth was in shorter supply than usual and thus more expensive. Women were moving toward more practical, shorter skirt lengths and fewer undergarments. They were also cutting their hair short. Tailored clothing gave way to store-bought, off-the-rack garments, changing the silhouette. Children's clothing was affected similarly, with lengths shortening and skirts losing volume to allow more efficient use of cloth.
Boy's Clothes
Boys in 1915 wore mostly short pants paired with white or light-colored, button-down shirts and sometimes jackets matched to the pants. Suspenders, not belts, held up short pants. Poorer boys and boys who worked more often wore overalls, like their fathers. Boys also wore knickers or knickerbockers, especially as play wear.
Girl's Clothes
In 1915, girls primarily wore dresses or skirts that ended at about the knee, paired with knee or ankle socks. Dress yokes dropped to the thigh. Girls commonly wore middy blouses -- shirts with a sailor-style collar and loose waist -- and skirts to school. The growing popularity of sailor-style clothing for girls seems to have led to a corresponding drop for similar boys' clothing. Fabrics moved from fragile, easy-soiling muslins of the previous decade to prints and ginghams in bright colors.
Shoes
Girls commonly wore single-strap, Mary Jane-style shoes, usually made of black patent leather. In cooler weather, they might wear lace-up oxfords. Girls also still wore the old-fashioned, button-up ankle boots commonly associated with the 1800s, but they were almost always dark colored on the bottom with a top side and ankle section in a lighter color. Boys primarily wore lace-up oxfords and high-top, lace-up boots similar in style to modern work boots. Unlike children's shoes of a few decades back, almost all children's shoes in 1915 were sensible in style, made of leather and in plain, well-wearing neutral colors ranging from beige to black.
Undergarments and Nightwear
Boys wore one-piece underwear, split in front like today's briefs for convenience, with light, sleeveless versions for summer wear and heavy, long-sleeved versions for winter. Girls' underwear was more complex, with short pantalets, a chemise, and sometimes a camisole. Nightwear changed little, with both sexes wearing a shift-like gown, adding a sleeping cap in the winter.