Save the Children said artillery shells hit the southern wing of a health centre it supports in the town of Maarat al-Numan in jihadist-held Idlib province.
A doctor was among the wounded, it added, without specifying who was behind the attack.
The Britain-based Syrian Observatory for Human Rights said regime artillery attacks hit the town of Maarat al-Numan on Thursday, without specifying casualties.
"The Maraat Al Numan health compound served up to 300,000 people living in the area, with on average 200 visitors daily, offering critical help to hundreds of children and their families," Save the Children said in a statement.
It said the attack put the facility out of service, but that the full extent of the damage was not yet clear.
"Yet again, civilians are paying the price of a long, deadly conflict," Save the Children spokesman Amjad Yamin said.
More than 400,000 Syrians have been displaced by violence in the Idlib region since the end of April, the United Nations says.
A four-months-long Russian-backed regime offensive has killed nearly 1,000 civilians, according to the Observatory.
A ceasefire announced by Moscow on August 31 has reduced air strikes, but skirmishes on the ground persist.
"The area has been relatively calm since the announcement of the ceasefire, and the displaced populations had started returning, now this may send them back," Yamin said.
The war in Syria has killed more than 370,000 people and displaced millions since erupting in 2011 with the repression of anti-government protests.
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