Cronus decided to eat up the children because he didn't want them to take over being lord of the titans. Cronus ate up Hestia, Demeter, Hera, Poseidon, and Hades. But Rhea hid Zeus on Crete.
As Zeus grew up, He forced Cronus to drink a potion that would vomit the children out. After that, The Cyclopes gave Zeus lightning and Zeus's masterbolt, Poseidon a Trident (a three pronged spear.), and Hades a helmet of darkness.