Sources and Uncertainty
This is a genuinely thin commemoration: no extended hagiographic life of Eleutherius and Leonidas has survived. The OCA Synaxarion entry is essentially complete in a single sentence, relating only that the martyrs, along with many infants, were cast into a fire at a youthful age during one of the persecutions against Christians.
External references add little. The Eastern Orthodox liturgical listings for August 8 give the place as Constantinople and, in one case, assign the martyrdom to the 4th century; the saints are also noted among the commemorations falling within the Afterfeast of the Transfiguration. No source supplies the name of the persecutor, a specific year, or further biographical detail.
Because the record is so spare, this profile deliberately remains short. The Church venerates the group as a single named commemoration, holding the witness of these martyrs and the infants who died with them even where their individual histories are lost.