UN Secretary General Ban Ki-moon on Friday called the 20-year-old Convention on the Rights of the Child as a beacon and guide to protect "the youngest and most vulnerable members of society".
The convention was signed into international law in 1989 by the UN General Assembly and it took effect one year later following ratifications by governments.
"Its influence has been profound," Ban told an audience at UN headquarters in New York. "It has become history's most widely accepted international human rights treaty."
"But realising the rights in the convention remains a huge challenge," he said, echoing the views of those organisations involved implementing the convention's provisions.
The UN Children's Fund (Unicef), tasked with carrying out the convention, said child mortality rates have dropped by 28 percent since 1990.
An estimated 8.8 million children under age five died from various diseases in 2008 compared with 12.5 million in 1990.
The 54-article convention provides children with rights and protection ranging from child labour to sexual exploitation. It provides protection from being drafted as child soldiers in conflicts, prostitution and domestic servitude.
Unicef said "considerable progress" has been made since 1990 to provide education, healthcare programmes as well as access to safe drinking water.
The convention has been ratified by 193 governments, making it an international document that has received the most ratifications in the world.
But Unicef executive director Ann Veneman said in launching a report on the 20th anniversary of the convention that children's rights are still far from being assured.
"It's unacceptable that children are still dying from preventable causes, like pneumonia, malaria, measles and malnutrition," she said.
The anniversary was being commemorated worldwide to highlight the importance of upholding the convention's provisions, Unicef said.