"Children make up for 30 percent of the civilian casualties," said the UN children's agency UNICEF, adding that the toll was based on deaths which it was able to verify and was likely to rise.
"The number of child casualties during the last 48 hours may rise as a number of incidents are pending verification," it said in a statement.
UNICEF stressed that its figures are "cross-checked to the best extent possible in the current situation... Subject to change based on further verifications."
The toll breaks down to 187 boys and 109 girls, with at least 203 of them under the age of 12.
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More than 1,650 Palestinians in the Gaza enclave have been killed since Israel launched Operation Protective Edge last month, aimed at halting militant rocket fire.
Most of those killed have been civilians, and more than 8,900 have been wounded.
Israel has itself lost 63 soldiers, while two civilians and a Thai worker have been killed inside the Jewish state.