Nearly 1,500 children recruited by Yemen's Houthi rebels died in fighting in 2020, and hundreds more the following year, the UN says.
In a report to the Security Council, experts said rebels were still recruiting children, using summer camps and a mosque to spread their ideology.
They added that air strikes on the rebels by Saudi-led forces were still inflicting many civilian casualties.
More than 10,000 children have been killed in the war that began in 2015.
Tens of thousands of adults have also died as a direct result of the fighting, with millions displaced and on the brink of famine.
In the 300-page report, the panel said it had received a list of 1,406 children recruited by the Houthis who had died on the battlefield in 2020 and 562 between January and May the following year.
"The children are instructed to shout the Houthi slogan 'death to America, death to Israel, curse the Jews, victory to Islam'," the Associated Press news agency quoted the four-member panel of experts as saying.
"In one camp, children as young as seven years of age were taught to clean weapons and evade rockets."
It called on all parties "to refrain from using schools, summer camps and mosques to recruit children" and recommended sanctions for anyone who did not.
It also said the rebels, who control the capital Sanaa, were using a complex network of global intermediaries to source critical components for their weapons systems to get around a UN arms embargo.
"All military and paramilitary forces loyal to the Sanaa-based authorities fall under this definition," it added.
There has been an upsurge in fighting following a Houthi drone attack on government allies the United Arab Emirates two weeks ago.
On 21 January an air strike on a detention centre in the north-western Houthi stronghold of Saada killed more than 70 people, prompting the UN and US to call for an end to the escalation.