The Britain-based Observatory said 20 children had been killed between midnight on Saturday and Sunday afternoon, with 17 more killed between Friday and Saturday night.
The deaths came in regime shelling and air strikes across the country, though most took place in the northern province of Aleppo and northwestern Idlib, Observatory director Rami Abdul Rahman said.
Many of the deaths came in raids involving the use of explosive-packed barrel-bombs, a weapon that has been criticised by rights groups as indiscriminate.
In northern Aleppo province, another five children and three adults were killed in an air raid in the west of the province, it added.
In the capital Damascus, meanwhile, regime planes continued to pound the eastern rebel-held district of Jubar, where the government began a fierce offensive earlier this week to wrest back control.
The Observatory said at least 15 air raids hit the district today, but there were no immediate details about casualties.
In mid-August, the army took Mleiha some 10 kilometres southeast of Damascus, and capturing Jubar would allow a two-pronged advance on Eastern Ghouta.
Rebels arrayed around the capital regularly fire mortar and rockets into Damascus.
More than 190,000 people have been killed in Syria since the conflict began there in March 2011, according to the UN.
