Voice is the most discussed and least explained thing in writing advice. In children's books it's unusually important, because the books that last are the ones a child can hear — and it's more concrete and more learnable than the mystique around it suggests.
What voice actually is
Not style, exactly, and not vocabulary. It's the sense of a particular person telling you this, with a particular attitude toward what's happening.
Think of a children's book you know well. You can probably hear how the narrator feels about the characters — amused, conspiratorial, fond, dry, mock-serious. That attitude, consistently held, is voice.
Where there's no attitude, the prose is competent and anonymous, which is what most first manuscripts sound like.
Why it matters more in this genre
Children's books are read aloud, and often many times. A voice is what makes the twentieth reading tolerable for the adult and still enjoyable for the child.
It's also what makes a child request one book over another. Young readers are extremely responsive to the feeling of being spoken to by someone specific.
Where first-time authors lose it
Writing down to the reader. Simplifying everything, explaining jokes, softening anything sharp. The result is bland, and children are much better at handling complexity than they're given credit for.
Imitation. Writing a version of whoever you loved as a child. Useful as practice, limiting as a destination, and usually audible.
Over-editing. The oddities get smoothed out, and the oddities were the voice. Many manuscripts get worse between draft three and draft eight for this reason.
A narrator with no opinion. Reporting events neutrally. Neutral is the absence of voice.
How to find yours
Read it aloud. Non-negotiable in this genre. The sentences you stumble on are wrong, and the places where you naturally add emphasis are telling you where the voice wants to be.
Keep the specific over the general. A dog has no voice. A dog who had never once come when called does.
Leave in what amuses you. The line you suspect is too strange is frequently the best one in the manuscript. Writers cut those first, and they're what makes a book sound like someone.
Write the same scene three ways. Once dry, once warm, once conspiratorial. You'll recognise which one is yours, and that recognition is more useful than any amount of theory.
Trust the reader. Explaining less creates voice almost automatically, because confidence is audible.
On writing for children specifically
The useful stance is talking to a child you respect — not performing for one, and not instructing one.
The books children keep tend to be slightly subversive, a bit funny about adults, and willing to acknowledge that life is unfair. That's not a trick of voice; it's an attitude the voice carries.
How long it takes
Most writers find it in the third or fourth project, not the first. It arrives through volume rather than through analysis.
Which is the one honest piece of advice about voice: it isn't found by thinking about it. It shows up in the writing you do while you're busy trying to tell a story.