One thing that I don't think gets talked about a whole lot among writers is this: why is it--or should it be--different to write for children than for adults?
There are a lot of possible answers to that question. Some of them are obvious; some less so.
An obvious answer is that children aren't adults; not only that, but the picture book crowd isn't ready for easy readers and the easy reader crowd isn't ready for MG fiction. The words used, the sentence structure, the presence or absence of pictures--all of those things will be different depending on your audience.
And that's true. But when you are writing for middle grade readers, kids who are already capable of reading chapter books, whose vocabularies are growing and whose ability to read complex sentences is well underway, what makes their books different from, say, a young adult novel or a book written for adults?
I think the answers can be summed up in three words: what, how, and why.
The "what" refers to: what are you writing? Even today, when there is more pressure on writers to write edgy fiction for younger readers, there are some topics that are seen as inappropriate for children. There are mystery novels for young readers, and I can think of a couple of examples of such novels that are murder mysteries, but almost no one would think it appropriate to write a novel for a middle grade reader in which the protagonist turns out to be a charming and likable serial killer. Similarly, while adult relationship issues including divorce will crop up on children's fiction, even the proponents of edgy fiction would not write a work of MG fiction featuring an eighth grade prostitute as the main character and narrator (whether such a work can be written for adults in a way that is conscience-raising and serious as opposed to exploitative and encouraging of pedophilia is beyond the scope of this blog).
But even if the "what" has been satisfactorily settled, the next question is "how?" as in, "How will you present this material in a way suitable for children?" To go back to the first question--suppose that you wish to write a murder mystery for middle schoolers; how will you go about doing it? If your first thought is of crime scene TV dramas where bloody corpses litter the scenes and forensic teams gather gory evidence in great detail, you're not really planning a children's book. On the other hand, if you are writing an easy reader for children in the six-to-eight range, you probably want to tone down the mystery to some missing cookies or something else suitable for the littlest readers. The way you approach the material is important, and you will find that restraint is more necessary when you are writing for children. I have, on occasion, written a scene only to realize that it is too dark or too scary for my youngest potential readers, at which point I will rewrite it with a more careful thought process centered around what children are expecting.
The final question, the "why?" refers both to individual parts of a given work and to the whole book. What message do you want children to take from your book? What purpose does a particular plot point serve? Most writers for children do not want to write books that are cynical and despairing, for instance, no matter how many adult books will contain those kinds of elements in them.
How do you think children's books differ from books written for adults? There are many more possibilities; I've covered only a few.