David Beckham speaks at a press conference to mark his 10 years as a UNICEF Goodwill Ambassador, at Google's headquarters in central London, February 9, 2015.[Photo/Agencies]

Beckham also asked his fans to back his call to world leaders to put children first as they negotiate new global development goals.

The launch of "7" comes after one of the worst years for children on record, UNICEF said.

More than 15 million children were exposed to extreme violence in conflicts in 2014 and millions more were hit by natural disasters.

The Ebola epidemic in West Africa has also left thousands orphaned and many more out of school.

Worldwide, some 168 million children are engaged in child labour and many others are victims of sexual violence, trafficking and female genital mutilation, UNICEF said.