Rescue worker Almoustapha Alhacen said the bodies - 52 children, 33 women and seven men from Niger - were found on the route from the northern mining town of Arlit to the Algerian border.
Many were in an advanced state of decomposition and had been partly devoured by animals, probably jackals, he said.
Northern Niger lies on a major corridor for illegal migration and people-trafficking from sub-Saharan African into North Africa and across the Mediterranean into Europe.
Most of those who make the perilous journey on ancient open-topped trucks are young African men in search of work. Rescuers said the doomed convoy of women and children was puzzling.
"It's the first time I've seen anything like it," Alhacen told Reuters by telephone from Arlit. "It is hard to understand what these women and children were doing there."
Rescuers found many writing slates in the luggage, suggesting the children may have been students in a Qu'ranic school being taken to Algeria, perhaps to beg, Alhacen said.
