I write YA (This will not be a surprise to anyone who reads this blog.)
I also write MG. (Also, not likely a surprise.)
Voice is important in both of these age groups because no one (kids especially) wants to be talked down to. And kids are experts at detecting fakers. (Those who toss a few text phrases into what could otherwise be the narration of a 36 year-old divorcee.. LoL!) But as important as it is, voice is also extremely tricky in these instances.
Adults don't think like kids. Adults don't speak like kids. Adults filter the world through a layer of experience that kids haven't lived long enough to experience, and there are serious chemical changes that happen during childhood and adolescence that separate the reasoning capacity of those who are and aren't biologically mature, and sadly, too many adults forget this the second they turn 22. (Most neurologists say that emotional maturity kicks in when a person is in their 20's.)
An adult writing for children or teens is essentially an exercise in translation. It doesn't matter that you speak the same regional dialect as those younger than you, you're still not speaking the same language, and if you want to succeed in writing something a kid of any age won't roll their eyes at, then you have to learn the dialect.
By this point, you're probably wondering what any of this has to do with the Bunny or Juice referenced in the post title. Well, it's a case in point.
I was at the supermarket and there was a harried mother pushing a cart with a girl in it who I'm going to guess was in the 3-4 year-old range. This little girl was being very well behaved, and at one point, she stopped chattering and playing with her doll to ask: "Mommy, can we get some Bunny Juice?"
The mother gave her an instant 'yes' and kept going. It was a cute moment.
Later, I saw this same mother and girl in the dairy section. Mom was still shopping; the girl was still behaving herself, and once again, she asked "Mommy, can we get some Bunny Juice?"
Again, Mom said 'yes', but this time, you could tell, Mom was getting frantic. You see, she'd promised the girl some Bunny Juice, but had no idea what it was. She scanned the milk cartons and juice jugs, searching for any clue as to what Bunny Juice might be. Maybe carrot or some brand the girl had tried at day care that had a rabbit on the label, but she couldn't find anything.
As I went on, there was another request from the girl, and another assurance that Mom would find this elusive drink.
When I was leaving, I spotted them again, in the checkout line. The girl had a container of Nestle Quik. She didn't know what it was called, only that it was a drink, which in her mind made it 'juice' and there was a bunny on the carton. Bunny Juice, simplified. (I'm sure I've missed a few things in this little story, but this is the basic version.)
This is the thing with a 'real' kids' voice. An adult would put something like that in the context of "Mom, can I have some of that chocolate milk you make with the powder? The one with the rabbit on it." if they didn't know the name of something.
A kid distills that into 'Bunny Juice'.
You can't force voice to sound right in a story any more than you can speak a foreign language and not have an accent at first. You have to work at it; you have to practice. And to do that, you need to listen. Listen to the people who are in your target readership and get a feel for how they say things and how they interact with others. (*note*If you are writing for kids, this does NOT mean you should stare at or follow random children. Their mothers will report you as the 'creepy' man/woman to security.)
Read novels others have written. Watch the television shows geared toward that age group. If you can nail the voice, the rest of the story will be a whole lot easier to get across.