“I don't know what's wrong with these kids today!”
Sorry. Broadway moment. I'm talking about using kids as tools in our writing. Okay, that sounds really bad, but that's what it really is. Little kids are great tools to provide dialogue, get your more mature characters thinking, and provide more internal conflict. Plus kids provide the cute factor for the reader, and they make your characters care about someone that can't take care of him/herself. And putting kids in dangerous, potentially lethal situations, while a pretty cheap emotional ploy, works.
But I'm not here to talk about putting kids in dangerous situations. Well, maybe a little—I'm not above manipulative writing on occasion. When I started thinking about this post, I was thinking more about that old(ish) show, 'Kids Say the Darndest Things.' It didn't happen very often, of course, but on occasion the kids on that show would say something very apt, or something that would just make an adult stop. It's because kids are innocent, and don't really know those unspoken societal laws we old fogies have ingrained in us. When confronted with that childlike honesty, how your characters respond is a quick and cheap way to show us a serious part of their personality.
They can also increase conflict in a very serious manner, especially when you have a romance or action-based plot. Think about it. If your romantic interest has a kid and your main character is reluctant about children, you have instantaneous conflict, and a chance to soften the main character's personality through the course of the plot. In action-based books, unless your main character is a heartless asshole, when you somehow or another put the kid in danger, your characters are going to have to go out of their way to keep the young one safe from the marauding demons or what have you.
Now, when you do put kids in dangerous situations, don't waste that reader reaction, either. One of the easiest things to do is make your reader hate someone or something for trying to harm the little kid. As soon as, say, your evil sorcerer tries to sacrifice a child (or, better yet, a schoolhouse full of children) to the Nameless Dark One for ultimate power or something, your reader is going to call him an asshole and want revenge.
So yes, when it comes to literature, children are really good multitask tools. Simple as that.