In its annual report published on Tuesday, Save the Children, said the overall situation for children has improved in 173 of 176 countries since 2000, but one in four under the age of 18 are still being deprived of their right to a safe and healthy childhood. Those living or fleeing conflict zones are among the most vulnerable, it said.
"While progress has been remarkable, millions of children continue to be robbed of a childhood. We now need to continue to push to reach every last child and ensure they receive the childhood they deserve," Helle Thorning-Schmidt, the organisation's CEO, said in a statement.
The study found nearly 31 million children forcibly displaced - an 80 percent increase since 2000 - and an estimated 420 million more living in conflict zones.
Rukaya Sarumi, advocacy manager at Save the Children, said it was a "very clear area of concern".
"In conflict areas, finding ways to ensure that children are healthy, are able to attend school, their life is free from violence and that their rights are protected becomes even more difficult," she told Al Jazeera.
The Syrian civil war, now into its ninth year, contributed largely to the increased displacement.
Afghanistan, Central African Republic, Democratic Republic of Congo (DRC), Iraq and Mali were ranked the five worst conflict-affected countries for a child.
Over the past two decades, the study found a 49 percent decline in under-five mortality rate, 17 percent decline in child homicide rate, 33 percent decline in children out of school and a 40 percent drop in child labour and 25 percent in child marriages.
Singapore, Sweden, Finland, Norway and Slovenia were ranked the top five countries for children.
Sarumi called on governments to continue their efforts to reduce "childhood enders" and "push for the implementation of UN's Sustainable Development Goals (SDGs)".
"We really continue to call for continued investment and political will at the national level and continued action at the international level, particularly in the areas that we have seen less progress in," she said.
Compared with 2000, Save the Children documented 11 million fewer child brides today, noting significant progress in countries like India, where child marriage declined by 51 percent.
The group attributed the decline in India to better education for girls, increased public awareness, community-based interventions and proactive government investment.
With three million fewer teen births worldwide per year, the adolescent birth rate also fell by 22 percent.
In a landmark case last year, India's Supreme Court outlawed sexual intercourse with a wife aged between 15 and 18.
But despite the decrease in the global percentage of child marriages, more than 150 million additional girls will be married before their 18th birthday by 2030, the United Nations International Children's Emergency Fund (UNICEF) said in a report last year.
Rachel Yates, interim executive director of Girls Not Brides, an umbrella body of civil society outfits fighting to end the practice, said child marriage is a "complex issue" and there is "no single solution" to eradicate it.
"We need to address head-on the belief that girls are not as valuable as boys, and that their only role in society is to become wives and mothers," she told Al Jazeera.
"Only when we tackle child marriage head on will all girls are able to live happier, healthier lives of their choosing."