Five bodies were recovered and one passenger was missing, presumed drowned, following the capsize on Thursday.
Some of the 21 survivors told aid workers there had been 27 people from eight Syrian families on the boat.
The Phoenix, a rescue boat run by the Malta-based NGO Migrant Offshore Aid Station (Moas) and the Italian Red Cross (CRI), recovered the corpses of two women, one man and the baby while the five-year-old's body was picked up by a fishing boat, the organisations said.
Details of the tragedy emerged as the world's attention has been re-focused on the crisis in Syria by this week's poignant pictures of the bloodied, dust-covered face of four-year-old Omran following the bombing of his home in the war-torn city of Aleppo.
Moas co-founder Regina Catrambone said the latest deaths were a tragic reminder that hundreds of migrant children continue to perish at sea a year after toddler Aylan Kurdi's body was washed ashore on a Turkish beach last year.
'Very sad and frustrating'
“It is very sad and frustrating to witness the tragic loss of life at sea, especially that of such a young child,” she said.
“It is time for the international community to come to terms with this reality and to implement safe and legal solutions for the most vulnerable among us to avail themselves of the rights and protections they are entitled to.”
Children represent a growing proportion of the migrants trying to reach Europe by sea from Turkey and North Africa – 27 percent of all arrivals in the first six months of this year were minors, according to the UN refugee agency.
Italy's coastguard said the five deaths occurred on a day when they supervised the rescue of 534 people in 11 different operations off Libya.
Nearly 100,000 migrants have landed at Italian ports this year, roughly in line with the level of arrivals in the same period in 2015. Numbers arriving in Greece have dropped sharply since an EU-Turkey deal in March that is aimed at stemming the flow.
More than 3,000 people have died trying to reach Greece or Italy since the start of the year, a 50 percent rise on last year.
It is relatively rare for Syrians to try to reach Italy via Libya. Over 90 percent of the migrants arriving on Italian shores this year have come from sub-Saharan Africa.