The coalition backs the nationally recognised Government and has been at war with the Houthis since March 2015. The rebels control much of northern Yemen, including the capital, Sanaa.
Al Masirah TV aired dramatic images of wounded children, their clothes and schoolbags covered with blood as they lay on hospital stretchers.
The International Committee of the Red Cross said on Twitter that its team at an ICRC-supported hospital in Saada received the bodies of 29 children, all under 15 years old. It also received 48 wounded people, including 30 children, it said.
The attack took place in the Dahyan market in Saada province, a Houthi stronghold.
Guterres condemned the coalition airstrike and called on all parties to spare civilians and "to respect their obligations under international humanitarian law, in particular the fundamental rules of distinction, proportionality and precautions in attack", UN deputy spokesman Farhan Haq said.
In the hours after the Thursday attack, airstrikes hit Sanaa, and sounds of the blasts reverberated across the city's southern and western neighbourhoods.
It was not immediately clear if there were any casualties in those strikes.
Yemen's stalemated, three-year war has killed more than 10,000 people, badly damaged Yemen's infrastructure and crippled its health system. The coalition faces widespread international criticism for its airstrikes in Yemen that kill civilians.
Impoverished Yemen, on the southern tip of the Arabian Peninsula, is now in the world's worst humanitarian crisis, with more than 22.2 million people in need of assistance.
Last week, Yemeni medical officials said the coalition conducted airstrikes in the rebel-held port city of Hodeida, killing at least 28 people and wounding 70.
But the coalition denied carrying out any attacks in the city, saying it follows a "strict and transparent approach based on the rules international law".
The fight for the port of Hodeida, a key lifeline for supplies and aid for Yemen's population on the brink of starvation, has become the latest battleground in the devastating war.
The Iran-aligned Houthis regularly fire into Saudi Arabia and have targeted its capital, Riyadh, with ballistic missiles. They say their missile attacks on the kingdom are in retaliation for air raids on Yemen by the Western-backed coalition.
- AP