As our Western culture society became more codified, or rather solidified into the general forms it still largely bears today, so too our notions of child rearing, and our attitude towards the children we raised, and what we expected of them, solidified. For hundreds of years the expectation was that children, once past infancy, would be active contributors to the life of the family, working and maintaining the space they had. So, the reality became that we started thinking of children as small versions of adults. They were expected to behave with the same manners, and do, as much as possible, the same work. In the upper classes the constraints on children were different, but no less great, since they were expected to master a huge range of education. Though it was a different education for women, from that of men, the intensity of it, and the social expectations that arose from it, bore the same strength.
It wasn't till the end of the 18th century that the position of children began to shift away from this mini-me status. Revolutionary philosophical writers like Jean-Jacques Rousseau, and Voltaire presented for the first time, the notion of childhood as an honorable, and entirely separate state of being. And so, by the middle of the 1800s, clothing for children was taking a different direction. Girls dresses, and boys suits were structurally, and in decoration, different from that of adult clothing.
How is Attire involved in this process? Why quite simply, because for good or ill we create instant opinions about each other and our relative places in the world based on what we see at first glance. You can think that's a bad thing, or a good one. But whatever you think , its still a truth about us.