During World War I began most of George and Mary's children were teens or young adults. I think that once teens the children started living separately from their parents -naval and boarding schools, John living primarily at Sandringham.
Going back to the mid to late 1800s, the British royals showed great interests in the new-ish technology of photography, with some like Alexandra of Denmark became amateur photographers. Going back to Victoria and Albert, the the strategy was to use imagery of the overall royal family including the children, whether painting or photos for public relations.
I thought all of the George V children were fairly photogenic, not just their looks but probably the staging as well, and if there was a standout it was Edward / David, they were certainly better looking than the Edward VII children. Also I think as mass communications including photos became more present, the royal family took advantage of the phenomenon to make them visible.
An excellent observation!
