The Libyan Red Crescent said the bodies of four children aged between five and 10 had washed ashore west of the capital Tripoli on Wednesday after a migrant boat sank.
"We were informed (Wednesday) of a boat that was sinking on the high sea with 30 people on board" in the Zawiya area, the Red Crescent's Hassan Mokhtar al-Bey told AFP.
He said rescue workers discovered three of the bodies near Zawiya, around 45 kilometres (just under 30 miles) from Tripoli, and the fourth a few kilometres further west.
The fate of the other passengers, their nationalities and point of departure were as yet unknown.
War-torn Libya has become a key embarkation point for migrants, mostly from sub-Saharan Africa, attempting death-defying voyages to Europe to flee conflict and poverty at home.
Blankets, including those for a baby, lay on the rocks near where the bodies were found, an AFP correspondent said.
Bey said his team expected to find more corpses, adding that those of the children had been taken to the Zawiya morgue.