In video Infant killedin Gaza
In video Palestinian rockets into Israel
AnalysisSaving the Palestinian sinking boat
"In the last three days at least 19 children have been killed ... it's a real concern for all organisations here," she said.
"Most of the time, when we go into the field and talk to the children about their fears and concerns, they are always afraid of a new [Israeli] invasion to the Gaza Strip - and obviously the current situation is just ... what they fear."
She said the number of children being hospitalised was increasing "day after day".
Al Jazeera's Nour Odeh on the situation in Gaza
"Israel puts itself above the law because the international community is always silent," he said.
Israeli incursion
The main Israeli operation began before dawn on Saturday.
Israeli tanks, supported by helicopters, pushed into the town and refugee camp of Jabaliya in the northern Gaza Strip and continued the assault throughout the day.
More than 150 people were wounded in the operation, Dr Muawiya Hassanein, the head of Gaza's emergency services, said.
At least 16 of the Palestinians killed were fighters - including 13 from Hamas, two from Islamic Jihad and one from the Popular Resistance Committees.
Tariq Dardouna, a Palestinian resident trapped in his house in east Jabaliya, told Al Jazeera that Israeli forces were targeting civilians.
"The Israeli army opens fire at everything in our area, including children and houses. There are injured children bleeding inside their houses," Dardouna said.
'Possible escalation'Ehud Barak, the Israeli defence minister, has said that Israel needed to prepare for a possible escalation of military action in the Gaza Strip."We will continue our action with all our strength and we need to prepare for escalation, because a broad ground operation is real and tangible," he said. One aim of any large-scale incursion into Gaza would be to "weaken the Hamas rule ... in the right circumstances even bring it down," he said.