After my post on not being nostalgic for college, I feel compelled to say that I do sometimes get nostalgic for my childhood. For years, this was not the case. For years, I never thought about being my childhood, because the trauma and unhappiness of college had, somehow, effaced all of that. But the process of writing all these young adult novels eventually brought it back.
It's weird. It's not that you recover a memory and then go on to write a book. It's that you write a book and the process of writing entails recovering the memory.
And it's not even a memory. To call it memory would be to misrepresent it. What you recover, when you write these books, is the feeling of being a child. And let me tell you, it's a pretty powerful feeling! Things are so important! Emotions are so immediate! You can write about falling in love with zero irony. You can write about becoming a high school valedictorian as if it's a matter of deathly importance. This would not be possible with an adult protagonist! But when you're writing a child, it's not only possible, it's necessary.
Kid's literature is full of precocious and worldly-wise teens who are probably more intelligent and witty than any teen ever was. However, even these kids don't escape from the drama of being young. In many cases, they're more susceptible to it than anyone. It's a trite example, but take John Green's protagonists. They're incredibly intelligent (to the point where it's annoying), but they also feel things so deeply.
And I feel the same way about my books. 
Sometimes I write these books that're about these kids, and I actually resent my protagonists. It's an incredibly perverse feeling, and I can't explain it. But I hate them, a little bit, because they're able to feel things so deeply.
When people talk about being nostalgic for childhood, they sometimes talk about how care-free it was. That, to me, is crazy. Childhood is not carefree. It's true that childhood (for most kids in America) lacks adult cares: how to feed yourself, how to stay healthy, the fear of mortality, etc. But just look at kids. They're so emotional. They're always crying. They're always worrying. They're always agitated. They're not faking. Those are real emotions. And their experience of those emotions is, in many ways, much more extreme than an adult's.
I would say that, if anything, adulthood is more carefree than childhood. Because while the things we worry about might be bigger, we worry about them less. Partly that's a result of more knowledge (we have a better idea of the kinds of things that can happen) and partly it's a result of more freedom (true anxiety comes when you don't have much control over your fate) and partly it's just biology (our brains and hormone levels are more settled). But, for whatever reason, I think adults don't feel as deeply as kids do.
For many, that's a good thing. It seems like every book I read contains some reference to the author's miserable childhood. However, to the best of my recollection, my childhood was not miserable. I did get really depressed at the end of my senior year in college (as my mother once reminded me). And I was bullied a bit in middle school. But otherwise, I was pretty happy. My childhood was pretty aimless. I didn't do much. I didn't have boyfriends or girlfriends or go to raging parties or even participate in much in terms of extracurricular activities. I read lots of books, but I also mostly read the same books over and over. I spent an insane amount of time playing video games. But I wasn't unhappy. I had friends. I had hobbies. I had projects (I spent a lot of time working on my D&D campaign). And I had my vague ambitions (I wanted to be involved in space travel!) And that was all pretty satisfying.
Of course, I don't feel nostalgic for those activities. I feel like I've spent a lifetime playing video games and planning D&D campaigns, and I have no need to revisit those activities. But I do feel nostalgic for the sense of aliveness that I felt back then.