The 33-year-old model-and-actress has teamed up with the fashion guru to mark 100-year anniversary of the charity - which aims improve the lives of children through better education, health care, and economic opportunities - and the launch of their new Stop the War on Children campaign.
And Poppy has revealed that she was inspired to create the organic cotton t-shirts - which feature an illustration of a crescent moon face alongside the words 'The Future is Now' - to ''encourage'' people to ''do something good'' and support the charity in a fashionable way.
In a statement, Poppy said: ''Having visited many Save the Children programmes over the past four years - most recently Za'atari refugee camp in Jordan a fortnight ago, I've met with some of the most inspiring and amazing children who, despite adversity and conflict, have not only survived to tell their stories but are now thriving and full of hope for a better future.
''It's because of these children and their daily fight that Alexa and I wanted to create something that would help make a change and encourage people to do something good.
''The future is full of possibility so join us and help make a change for the millions of children that need it the most around the world. Buy our t-shirts and wear them proudly!''
Alexa, 35, also insisted that she was ''drawn'' to the concept of a crescent moon because it symbolises ''children's dreams'' and the ''fight'' for their future.
She added: ''When coming up with the t-shirt design, I kept being drawn towards a crescent moon. To me it symbolises children's dreams, hopes and the promise of a new day.
''Sadly, not all children have the chance to enjoy such innocent childhoods as millions are battling through conflicts started by adults.
''But they should, and we need to fight for their futures and help them fulfil their potential by ensuring all children are off limits in war. Their future, our future, starts now. Please join us and help raise vital funds for Save the Children.''
The slogan was inspired by the charity's belief that every child should be able to make a mark on the world and hope for a better future.
Each garment will be priced at £25, and will be available in the colours navy, bright white and a limited edition run of canary yellow.
At least 30 per cent from each t-shirt sale will go to Save the Children and are available exclusively to order from www.savethechildrenstore.com from Thursday 16th May 2019.
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