How To Write For Kids?

Read a lot of children's books. That's the best way to figure out if your tone and subject matter fit with the audience.

By "kids", I'm assuming middle grade, not picture/chapter books?

There are a few things that generally indicate whether a story is for kids, teens, or adults (I'm sure, though, that if you looked hard enough you'd find exceptions):

→ Age of the characters: almost all middle grade books have a child MC; only some adult books do
→ Voice: middle grade voice is ridiculously hard to get down. It's not uneducated or full of "LOLs" (though it can be); it's one of those things that you know when you see
→ Plot elements: you can get away with childish stuff in MG, assuming it's well-written and makes sense. This book, for example. Or this one.

TL;DR: Read more books like the one you're trying to write.

Adding to what's above: The themes and plot have to appeal to kid readers. The reader has to be able to identify with the characters.

If you talk down, you lose them. If you go over their heads, you lose them.

Write like your telling a fairy tale to kids.
Add pictures and leave room for questions.
Then you can write another topic answering their questions.
Just my style.

Read a lot of children's novels, especially (but not exclusively) the sort you'd like to write yourself. You'll find the answers there.

As others have said, you have to read a LOT of books written for your intended audience. There is no other way to learn how to write for children. I write MG, but I have the advantage of loving to read MG. I read it as a kid, and I still read it for pleasure, and would continue to do so, even if I didn't want to write it.

I think it's almost impossible to write anything well if you don't also enjoy reading it.