One of the largest humanist organisations in the US has launched a new campaign aimed specifically at children and teenagers that tells them they are "a bit old for imaginary friends".

"Whether they already made up their minds to reject supernatural explanations, or are just questioning, it's time to make available an online resource that's built just for kids without God," Roy Speckhardt, executive director of the American Humanist Association, said in a statement.

"These kids may be from traditionally religious families, or from families like that of President Barack Obama, whose mother was a secular humanist. KidsWithoutGod.com will be a friendly online community for kids who might be too shy to ask an adult directly what it's like to be good without a god."

The campaign website tells kids who are "without God" that they are not the only ones. The ads feature young teens looking dismissively at a finger pointing at them from the clouds, presumably God, while the text reads "I'm getting a bit old for imaginary friends".

The new billboards by the American Humanist Association will be appearing on 140 Metro buses in Washington DC, and will cost the organisation $30,000. The group will also market the kids' campaign on big social media websites such as Facebook, Google and YouTube. The AHA was previously rejected by a website run by Disney and National Geographic because of the content of its ad.

"With the plethora of websites geared toward teaching kids about Christianity, Judaism, or Islam, we're pleased to add humanism to the discussion," said Speckhardt.

"Kids should know there's another way to learn about morals and values – it doesn't need to come from traditional religion."

The atheist campaign, which is also aimed at teens and parents, features sections on the KidsWithoutGod.com website explaining the humanist worldview. It states that its mission is to "bring about a progressive society where being 'good without God' is an accepted way to live life."

"We accomplish this through our defense of civil liberties and secular governance, by our outreach to the growing number of people without religious belief or preference, and through a continued refinement and advancement of the humanist worldview," the organisation said.