I read the book Children in the House by Karin Calvert. She has done alot in the history of children and anylyzing photos and paintings of them. The big thing she does is analyize clothing and the way children were thought of. Early in an American child's life children were molded to resemble little adults they weren't left to learn to crawl they were "taught" to walk without the oportunity to crawl because that was considered crude. They were also wrapped up tight (swaddled) to ensure they would grow up straight and tall. The pictures the importance of children were also shown in how they were positioned in the photographs.
I think it would be interesting to see where photos went after she stopped in the book. She goes to about the mid 1800's and mentions a little of the later part of the century but not too much on it.