As a child, I thought that the ages of the children were carefully left hard to measure, just as most of the daily life aspects were not quite there. It was more about Mary Poppins turning the world of the children inside out than about their ordinary lives,so the everyday was only referenced as an anchor. Mind you, this is me running on memory - no doubt I'll have an entirely different opinion when I can get the book. I do remember though, thinking about all the missing elements and trying to work out how my childhood related to theirs and thinking "This is not what the story's about." It was more surreal than real to me as a child.