Jacinda Ardern has responded to a comment made by a US presidential hopeful after she said her first priority in the Oval Office would be to call Ms Ardern and say "girlfriend you are so on" when it comes to which country is the best in the world for a child to grow up in.

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The American contender for the Democratic presidential nomination Marianne Williamson was asked during the NBC News Democratic Debate with other candidates what her first priority in the Oval Office would be.

"My first call would be to the Prime Minister of New Zealand, who said that her goal is to make New Zealand the place where it's the best place in the world for a child to grow up," Ms Williamson said.

"And I'm going to tell her, 'girlfriend, you are so on', because the United States of America is going to be the best place in the world for a child to grow up."

Speaking at an event today, Ms Ardern laughed off the bizarre challenge and said she thought it was "an unusual reference".

"There would have probably been a whole lot of Americans watching who would have said 'who?'," Ms Ardern said.

"If another country has the same ambition as New Zealand does for children, and their children, then that's a great thing.

"But I'm obviously sticking to our guns - our goal is to be the best place in the world for a child to grow up.

"It's all about making it great for kids."

A reporter then asked her whether New Zealand or the USA is currently the best place to grow up.

"Of course I would say New Zealand - of course I would say that - but that's not to say that there are things we couldn't do better," she said.

Ms Ardern said she thought the real experiences of children in the country was the most important indicator as to what is the best upbringing, rather than lists developed by international agencies.