For much of history, children were dressed in relatively simple garments that allowed for the fact that they couldn't yet dress themselves. As clothing became more structured, both boys and girls were dressed in skirts and dresses, which allowed for easier diaper changing and toilet training. The day a boy began wearing pants was a momentous occasion, and signaled that he was now a man-in-training.
With the power of pants came an understanding of manly responsibility, writes Jennifer Jordan in an essay on 17th-century masculinity. “The breeching ceremony stands out as one of the most significant milestones in a boy’s journey to acquiring manhood.” This seems to have been understood by even very little boys. Samuel Coleridge, the English poet and philosopher, described his five-year-old son Hartley being breeched in an 1801 letter. “He did not roll and tumble over and over in his old joyous way,” he wrote. “No! It was an eager & solemn gladness, as if he felt it to be an awful area in his Life.” These parties were usually held over a weekend at home, with relatives invited to stay. The pockets of Hartley’s breeches jingled with “a load of money,” Coleridge wrote, likely gifted to this fledgling man by visiting family members.
After his "breeching," a boy would spend his time in the company of men and other boys, while girls stayed near their mothers and learned the gentle arts of the home. Read about the historical tradition of a boy's transition to pants at Atlas Obscura.