Hesiod's Theogony lists the children of Phorcys and Ceto as the Graeae (naming only two: Pemphredo, and Enyo), the Gorgons (Stheno, Euryale and Medusa), probably Echidna (though the text is unclear on this point) and Ceto's "youngest, the awful snake who guards the apples all of gold in the secret places of the dark earth at its great bounds", also called the Drakon Hesperios ("Hesperian Dragon", or dragon of the Hesperides) or Ladon. These children tend to be consistent across sources, though Ladon is often cited as a child of Echidna by Typhon and therefore Phorcys and Ceto's grandson.
^Hesiod, Theogony, 295-297. Though Herbert Jennings Rose says simply that it is "not clear which parents are meant", Athanassakis, p. 44, says that Phorcys and Ceto are the "more likely candidates for parents of this hideous creature who proceeded to give birth to a series of monsters and scourges". The problem arises from the ambiguous referent of the pronoun "she" in line 295 of the Theogony. While some have read this "she" as referring to Callirhoe (e.g. Smith "Echidna"; Morford, p. 162), according to Clay, p. 159 n. 32, "the modern scholarly consensus" reads Ceto, see for example Gantz, p. 22; Caldwell, pp. 7, 46 295-303; Grimal, "Echidna" p. 143.
Fowler, R. L., Early Greek Mythography: Volume 1: Text and Introduction, Oxford University Press, 2013. ISBN978-0198147404.* Freeman, Kathleen, Ancilla to the Pre-Socratic Philosophers: A Complete Translation of the Fragments in Diels, Fragmente Der Vorsokratiker, Harvard University Press, 1983. ISBN9780674035010.