All were killed in their homes in the past week, the Norwegian Refugee Council said.
More than 60 children have been killed in recent airstrikes, according to the Gaza health ministry.
The Norwegian Refugee Council, which supports displaced people around the world, said 11 of them were taking part in its psycho-social scheme helping schoolchildren in the Gaza Strip to deal with trauma.
They were aged between five and 15 years old, the organisation said.
One of the children was the teenage daughter of Dr Ayman Abu al-Ouf, the head of the internal medicine department and coronavirus response at Gaza’s al-Shifa hospital, who died along with her father and 17-year-old brother in an airstrike in Gaza City over the weekend, according to the humanitarian group.
"We are devastated to learn that 11 children we were helping with trauma were bombarded while they were at home and thought they were safe," said Jan Egeland, secretary-general of the Norwegian Refugee Council.
"They are now gone, killed with their families, buried with their dreams and the nightmares that haunted them.”
Mr Egeland said: “We call on Israel to stop this madness: children must be protected. Their homes must not be targets. Schools must not be targets. Spare these children and their families.”
Heavy fighting broke out on 10 May when Gaza’s militant Hamas rulers fired long-range rockets toward Jerusalem in support of Palestinian protests against Israel’s heavy-handed policing of the al-Aqsa mosque compound, a flashpoint site sacred to Jews and Muslims, and the threatened eviction of dozens of Palestinian families by Jewish settlers.
More than 200 Palestinians have been killed in heavy airstrikes since then, including 61 children and 36 women, with over 1,440 people wounded, according to Gaza’s health ministry, which does not break numbers down into fighters and civilians.
In Israel, 12 people, including a five-year-old boy and a soldier, have been killed in the ongoing rocket attacks launched from civilian areas in Gaza.
Since the fighting began, the Israeli military has launched hundreds of airstrikes that it says have been targeted at the militant infrastructure of Hamas. Palestinian militants in Gaza have fired more than 3,200 rockets into Israel.
The fighting between Israel and Hamas is the most intense since 2014, but efforts to halt it have so far stalled.
The war has also seen an unusual outbreak of violence in Israel, with groups of Jewish and Palestinian citizens fighting in the streets and torching vehicles and buildings.
As the fighting drags on, medical supplies, fuel and water have run low in Gaza, which is home to more than 2 million Palestinians and has been under an Israeli-Egyptian blockade since Hamas seized power from rival Palestinian forces in 2007.
Tens of thousands of Palestinians have fled their homes so far.
Additional reporting by Associated Press
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