The children were on a field trip when their bus was struck at the Dahyan market in the Houthi stronghold of Sa’ada, the first stop of the day; 50 were killed and 77 injured, according to the Health Ministry.
Most of the children were inside the bus when the airstrike hit, Yahya al-Hadi, a local medic, said.
The International Committee for the Red Cross said a hospital it supports in Sa’ada had received 29 bodies of "mainly children" younger than 15, and 40 injured, including 30 children.
"(The hospital) is very busy. They've been receiving wounded and dead since the morning and it is nonstop ," Red Cross head of communications and spokesperson Mirella Hodeib said.
"Putting children in harm's way is horrific and deplorable and making them pay such a price is unacceptable," she said.
"We need blood," said Jameel al-Fareh, an emergency room doctor at Sa’ada's Al-Jumhuri hospital, calling on local people to donate blood to treat the wounded.
Ahmed al-Mansouri, the hospital's director, condemned what he called the "massacre of children".
UN Secretary General Antonio Guterres condemned the airstrike, calling for an independent investigation into the case.
"The secretary general condemns the airstrike today by the coalition forces in Sa’ada, which hit a busy market area in Majz district and impacted a bus carrying children from a summer camp," Farhan Haq, the UN chief's deputy spokesman, said in a statement.
He added that Guterres has urged an “independent and prompt” probe into the air raid.
The Saudi-led coalition, in a defiant statement, described the attack as a “legitimate action” to target missile launchers used by Houthi fighters to target the southern Saudi city of Jizan.
The military alliance said on Friday it will investigate the deadly.
On August 2, attacks on a hospital and a fish market in the strategic Houthi-held port city of Hodeida killed at least 55 civilians and wounded 170, according to the ICRC.
Aid agency CARE International noted that Thursday's strike came a week after the Hodeida bombardment.
"This latest air strike, only a week after the attacks on Hodeida city, demonstrates a continued disregard for human life and suffering," said Johan Mooij, the agency's country director in Yemen.
"It is beyond cruel; innocent children's lives have been lost."
CNN, AFP and Reuters contributed to this story.