They say that if you don't lean Left when you're young, you have no soul; but if you don't lean Right when you're older, you have no brains. Why can't you do both?
In popular media, it's always easy to tell the good guys from the bad guys. The good guys will be dashing, bold, witty, brave, loyal, gallant: in short, heroic, larger than life characters who make things better and fight for Truth, Justice, and that other thing.
The villains will be cowardly, sniveling self-serving distorted monsters; faceless Others like the Empire's stormtroopers, coweled Magical Neo-Nazis who would crush the Muggles beneath their pointed slippers, or otherwise disfigured, ugly monsters who deserve no better fate than to be dispatched in hordes before the main character's moral fortitude.
In an election, however, each side has a vested interest in telling you that the other guys aren't like you. That if you support Them, They will do terrible things to the world. They will let criminals loose on the world; They will destroy the Economy (which, frankly, no-one outside of a small elite even understands how it really works anyway) which will in turn destroy everything you've worked so hard to achieve in your long, hard struggle with life.
That's OK. That's how the game works: each political party has a story to tell you about how they see the world, and how they can make it a better place. They also have a story to tell you about how they're better qualified for the task of making New Zealand a kinder, fairer society than the other guys. They're entitled to their opinion, of course, and I'm not trying to say that the stories political parties tell you are all lies, slander, and empty promises. Of course not. You don't get to be on a political party list by being stupid, no matter what you might yell at the TV after the latest sensational scandal.
Instead, each and every political candidate sincerely and honestly believes that they're doing the correct thing for the country.
That's where you come in. A vote for a political party is, in essence, a way of saying "YES! I agree with everything you say you want to do! I have heard all of the arguments, and yours is the one that I believe in the most! Lead us into this better future you envision!"
A vote is not a way of saying, "Hey, I like most of what you said, but could you cut out A, B, and C? D's OK, but E could use some work. OK thanks bye."
Your vote is not subtle. Your vote is a sledgehammer, not a chisel. Your vote doesn't distinguish between National's vision of a brighter future where if you work hard, you can succeed, and National's stated intention of selling off just under half of NZ's state-owned assets. Unless you take the time to spell that distinction out in detail in a letter to your electorate MP, John Key, Bill English, et al, will take your vote to mean that you agree with every single thing they've said and done for the last three years, and are totally okay with everything they're telling you in the last four weeks before polling day.
So this is what "Think like a child, vote like a grown-up" means: It means, think about the stories that each party is telling you. Think about it in terms of, who's the bad guy, who's the good guy, is the good guy doing the right thing when they blow up the Death Star, dooming not only any Imperial soldiers, but anyone on board who was just trying to organise collective bargaining for the Endor Plumber's Union.
Once you've got the stories straight, stop and ask yourself: If they held elections every day for the last three years, would I have always voted for Party X? And am I prepared to vote the same way every day for the next three years?
Too Long, Didn't Read (TL;DR): Your vote is a signal to the politicians that you've been paying attention and agree with most of the stuff they've said and done in the last three years, and would like them to continue. Vote accordingly.