The photographs were used on Turkish news site Takvim on Boxing Day with a story quoting a Syrian civil defence official saying the children were killed during a bombing attack in the Idlib province.
The official, Mahmoud Dagim, said five children had been killed when Russian jets bombed Zikar school in the Jarjanas region. Four others were also said to have been killed.
The photographs appear to have first been posted online by a Syrian man, Seraj Aborass, who used his Twitter feed to blame Russian planes for a "massacre".
A string of casualty images purportedly from the bombing, including those posted online by Williams, were tweeted by an "activist media" Twitter account linked to government opposition forces.
Unicef has criticised Williams' use of the graphic photos, saying the images don't respect the rights and dignity of the children.
Spokesman Patrick Rose said Unicef was an organisation dedicated to protecting the rights of children throughout the world.
"We see it as a fundamental infringement of those children's rights," Mr Rose said. "But at the same time, we don't have the capacity to censor or edit private citizens' showing what they find on their individual explorations online."
Williams is not an official ambassador for the charity. Mr Rose said the rugby star's trip was to help him understand the crisis and to help raise awareness for the refugees' plight.
Mr Rose said it was obvious Williams had been "deeply moved" by his experience in Lebanon but didn't agree that the photos were the best way to help refugees.
"We [Unicef] are as disturbed as anyone when we see those images but we want to offer a positive framework for people to respond to that. We can't stop the war, we can't stop these things happening to people, but what we can do is help children by getting them clean water, by getting them counselling to help them deal with that trauma and to help them have a better future by keeping them in school."
Unicef had tried to contact Williams about the photos, but had not yet been able to reach him to voice their concerns.
"I don't think anyone would be happy about those kinds of images," Mr Rose said. "It certainly wasn't something that he consulted us about and they weren't images that he'd taken on the trip with us."
The public response to the photos has been mixed. Some have praised Williams for bringing attention to the crisis, and others say it could incite dangerous reactions and was disrespectful to the dead.
Williams' All Blacks and Chiefs teammate Liam Messam tweeted: "Good work, my brother."
Lebanon shelters more than 1.2 million refugees from the Syrian conflict, which began five years ago.
