Seven others were injured during the "violent thunderstorm" in the prefecture of Dabola in central Guinea, the ministry of security and civil protection said in a statement late on Thursday.
The victims took part in a baptism ceremony and were aged between 2 and 17 years, according to the statement.
The ministry did not provide any further details.
Baptism ceremonies in Guinea often take place outdoors, under trees or on the shores of lakes.
In July 2019, six children aged between four and 10 died after they were struck by lightning while making tea under a mango tree in north-eastern Guinea.
The storm began shortly before 7:00 pm (1900 GMT) in the town of Siguiri, close to the border with Mali, witness Mamadi Doumbouya, a local resident, told AFP.
He said eight children in total, accompanied by two of their mothers, were under a mango tree at the back of his house.
"I invited everyone to take shelter in my living room.
"The ladies rushed under my roof but the children stayed behind to make the last cups of tea," he added.
Lightning then struck the mango tree and when Doumbouya rushed out, all of the children were on the ground and unconscious, he said.
On the same day in 2019, a landslide hit a gold mine in the same area following storm, killing four people including a two-year-old girl and her mother.
"The victims were working in a former gold mine where mining was banned because of the risk of landslides" in the heavy rains, "but people were hiding to go to the tunnels", a Red Cross official had said.
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