Charity is the most consistently pictured of all the virtues, and the two objects that identify her are a flame and children. The flame, sometimes a burning heart held in the hand or a fire on the head, renders caritas as the theological virtue: love as something kindled rather than reasoned into. The children, usually two or three, one of them at the breast, render the same virtue as practical mercy, the giving of what one has to whoever needs it. Cesare Ripa's Iconologia, first printed in 1593 and the handbook every later programme worked from, gives both.
The doubling in the pictures follows the doubling in the doctrine that the atlas records elsewhere: charity stands in the list of contrary virtues against avarice, where it means generosity, and in the list of theological virtues alongside faith and hope, where it means the love of God and of the neighbour for God's sake. A painter needing one figure for both meanings needed two attributes, and got them.
The flame is the older half. It carries into Christian art the classical association of love with fire and joins it to Pentecost, and it is why charity is so often the only virtue in a cycle rendered in warm colour.
The atlas draws charity's emblem as the flame rather than the children, on the practical ground that a single flame reads at small size and a group of figures does not, and because the flame is the attribute that belongs to charity alone.