At the moment I'm trying to write a fantasy novel, and I think it's leaning more towards middle grade rather than children's or young adult fantasy, but I'd like to know if there are any particular clues I could look out for when deciding which I should class it as.
While I know the general age range for these genres (Children's (~8), middle grade (9~11), and young adult (12+)) and can think of some traits that tend to be in the writing of one or another, I can't help but wonder where one crosses over into another, and whether there are any clear cut distinctions between the themes/plots/stories for these genres. For example, while E. Nesbit's Five Children and It is very much a children's fantasy (it's light hearted, the children get themselves into trouble but not mortal danger, they learn from their mistakes), Susan Cooper's The Dark is Rising Sequence is described on Wikipedia as 'for older children and young adults' (more of a classic Light vs Dark, Good vs Evil, ancient-magic-awakening kind of story), so I assume that means it could be classed as either middle grade or young adult.
In the end, basically, I would like to ask: Are there any definite rules such as 'stories like this are always children's fantasy, and ones like this are always middle grade, young adult, etc.'?
There are no clear-cut distinctions. Children are different. One child might be reading at 6 what another wouldn't touch until 12. For example, King Matt the First is explicitly written for children (under 8). It deals with themes like death, war, responsibility, and it doesn't have a happy ending. I grabbed it off the top shelf in my room when I was 6, and I loved it. I was copying illustrations out of it, I badgered my mum for "King Matt on the Desert Island", and loved that too, even though it's even more tragic than the first one. Then I recommended it to a classmate, and she found it extremely traumatising - so much that she couldn't finish it.
The one definite rule I can think of is sex. Under-12s wouldn't have a reference point for physical attraction - it's not something they've ever experienced. They would know love - they see it, but sex would be alien to them, thus either wierd-eww, or just boring.
Another thing you'd want to avoid is philosophising. An adult can be engaged in a "discussion" with ideas presented in the book. A child doesn't have the experience to weigh their ideas against the book's: they do not yet have well-structured "ideas" of their own. A lengthy theoretical tract would thus bore them. Distopias go out of the window for the same reason - a young child would tend to accept things at face value.
Avoid using strong language. Having read the Three Musketeers when I was 10, I had a period of wanting to be like d'Artagnan, thus talking like d'Artagnan. I knew this wasn't a good way to talk, but I didn't care, because d'Artagnan was awesome. My mum wasn't happy.
I don't know why, but I remember as a child my world was very black-and-white: there were the Good Guys, there were the Bad Guys. There wasn't much grey. Good Guys sometimes made mistakes, and sometimes struggled to make the right decision, but ultimately I knew they would do the Right Thing. In fact, if I didn't know what the Right Thing was, it clearly was whatever the Good Guys did. That's something you'd probably want to keep in mind - children would regard the main character as an example, so you'd better make sure it's a good one.
Other than that, keep in mind that younger children would be less knowledgeable and less experienced. Don't talk down to them - they're not stupid, and they hate that, universally. But do give a bit more information than you normally would. In fact, children are curious, so don't hesitate to tell them things. I loved Jules Verne because (among other reasons) he "taught" me so much about geography, and strange people in far-away places, and marine life and whatnot. (Today I know most of that information is erroneous. The "Suck Fairy" strikes again.)
It's not a bad idea to have a child of your target audience's age as the main character - their experience and world-view would be similar, thus easier to understand and empathise with, but this isn't necessary. The Hobbit is a children's book about a 50-year-old. It is, however, easier, I think, for a child to empathise with a character who isn't "on top" in his environment - whether it's a child, a hobbit, a pet, a newcomer to anywhere. In this respect, they'd be closer to the reader, who is relatively small and weak, and aware of it.