The idea of kids in the position of authority, to me, is more or less just the same thing as how OPL made himself a virtual dictator in his idealized CWCville world. The world of Growing Around has even less definition, in comparison, because the supporting rationality for everything isn't important to Entard, it's all about what he wants. Which, basically, is the all-too-common fantasy of living as a child who is pampered by their adult parents but can freely make the rules how they please.
Nevermind the fact that such a society would crumble in a month because of the prevalence of religion in a large portion of the world, but if the cut-off age for world leaders is 12, which is before most schools start teaching more advanced mathmatics or more complex concepts of...basically anything that forms the foundation of any aspect of modern civilization.
Plus, the ones who would have the ability to actually do anything to improve society is still the adults. They would be easily be able to control the "leaders" with nothing more than appealing gimmicks on the level of how they sell toys in the real world we live in. The only case I know of where any kind of decent media setting where there was a child-led society was the Rankin-Bass Easter stop-motion feature (the one with the talking trains and the bait-switch gag involving the chocolate bunny), and aside from simply establishing that society existed, simply enough time did not pass to encounter the problem of what happened when the kids grew up before the movie ended.