It's pretty common in various media that characters don't grow any older unless they have some narrative reason to do so. Bart Simpson has been ten years old for twenty-five years now, and he's actually one of the newer ones.
At least L. Frank Baum explained that a lack of aging was part of life in the fairyland of Oz. Ruth Plumly Thompson amended that to no one aging unless they wanted to, but she never specified how someone would go about making the choice. A popular idea is that it's accomplished by making a wish on your birthday, but I always thought that was a little too cutesy.
Regardless, we're left with a lot of children who aren't growing any older, and I have to wonder what that's like. Do they become adults in child bodies, or do they never mature at all? I recall someone (I think it might have been J.L. Bell) posting to an Oz forum that brains tend to grow along with bodies, so a perpetual kid might not even be physically able to mature. Then again, we are talking about a land where animals can speak fluent English. And just because you're physically and mentally a child doesn't mean you can't learn new things, so long-time Ozian children could potentially have a lot of knowledge.
That's kind of how I think of someone like Dorothy or Ozma. They're physically young and still act it in some ways, but they still possess the knowledge that living for decades can give them. And they both have jobs of a sort, since Ozma rules Oz and Dorothy assists her.
Is it common for children to have to take work after living a certain number of years, or are their parents still willing to take care of them? We're never really told, but I have to suspect that many kids in Oz help out with their family's trade, or find work elsewhere. Kimbaloo from The Lost King of Oz, for instance, is a community of working children. Number Nine is physically twelve years old, but he has a job as assistant to the Wizard of Oz, and his older-but-younger sister helps out in Jenny Jump's style shop. Speaking of which, Number Nine and Sister Six are two of fourteen children, something I would imagine is rare in a land where nobody dies. But then, their family is rather odd even for Oz.
Overall, it appears that children in Oz get a pretty good deal, but I suspect some of them want to grow up anyway.