Perhaps this is picking up from the Octavius written by Minicius Felix in the late 2nd century, it's an apologetic text framed as a dialogue between Octavius (Christian) and Caecilius (Pagan).

Caecilius says that he has heard that Christians eat their children in their sacred meetings, Octavius responds that this is false, and perhaps a misunderstanding of the meal where they eat the body and blood of the Lord. He then compares the Christian practice of looking after children with the pagan practice of exposing infants, particularly girls.

The charge is also reflected in Tertullian, Ad Nationes 1.15, as part of a supposed initiation rite.

There's a full treatment of the idea of Cannibalistic charges in Andrew McGowan, 'Eating People: Accusations of Cannibalism against Christians in the Second Century', in Journal of Early Christian Studies (I don't have the volume/year on hand).