Doctors Without Borders, a Paris-based relief agency also known as MSF, said the children were killed yesterday in coalition raids on a school in Haydan, a town in rebel-held Saada province.
The coalition of Arab states has been battling the Huthi rebels since 2015 after the insurgents seized Sanaa before expanding to other parts of the country.
Ten days ago it acknowledged "shortcomings" in two out of eight cases it has investigated of strikes on civilian targets in Yemen that the UN has condemned.
"The site that was bombed... Is a major training camp for militia," he told AFP. "Why would children be at a training camp?"
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Yemen's government had confirmed to the coalition that "there is no school in this area", he said.
Assiri said MSF's toll "confirms the Huthis' practice of recruiting and subjecting children to terror".
"They... Use them as scouts, guards, messengers and fighters," he said, noting previous reports from Human Rights Watch on the rebels' use of underage recruits.
MSF spokeswoman Malak Shaher said those killed in the strikes on "a Koranic school" were all under 15.
She called on "all parties to take the measures necessary to protect civilians".
But Assiri criticised the organisation for overlooking the issue of child soldiers.
"We would have hoped MSF would take measures to stop the recruitment of children to fight in wars instead of crying over them in the media," he said.
The United Nation's children agency, UNICEF, also reported the attack.
It warned that "with the intensification in violence across the country in the past week, the number of children killed and injured by air strikes, street fighting and landmines has grown sharply".
Assiri sent AFP pictures of Huthi children carrying rifles and rocket-propelled grenade launchers.
Huthi spokesman Mohammed Abdulsalam said warplanes "targeted" children at the Jomaa bin Fadhel school, in what he called a "heinous crime".