According to Hesiod, Cronus swallowed his children because he learned "he was destined to be overcome by his own son":
These great Cronos swallowed as each came forth from the womb to his mother's knees with this intent, that no other of the proud sons of Heaven should hold the kingly office amongst the deathless gods. For he learned from Earth and starry Heaven that he was destined to be overcome by his own son, strong though he was, through the contriving of great Zeus. Therefore he kept no blind outlook, but watched and swallowed down his children: and unceasing grief seized Rhea.
Source: Hesiod. The Homeric Hymns and Homerica with an English Translation by Hugh G. Evelyn-White. Theogony. Cambridge, MA.,Harvard University Press; London, William Heinemann Ltd. 1914.
However, Cronus didn't only swallow his sons Hades and Poseidon; he also swallowed his daughters, Hestia, Demeter and Hera. Why? Is there, perhaps, a version of the myth that allows for the possibility that a female child will overthrow Cronus?
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