Kronos was the king of the Titans ( the Titans were a group of older gods who ruled before Zeus became king. They were led by Zeus' father, Kronos). He was very afraid that one of his children would try to murder him just as he had murdered his own father. He was so worried he started to EAT his own children just after they were born, much to his wife Rhea's horror!
After the birth of his sixth child, Zeus, Rhea played a sly trick on her husband Kronos. She gave Kronos a stone to swallow instead of the little baby Zeus. She hid baby Zeus in a cave and the young god was brought up by a goat.
When Zeus was older he asked to become his father's cup-bearer. Zeus put a special potion into his father's wine that caused Kronos to vomit up his five children that he had eaten. He also vomited up the stone which he was tricked into swallowing instead of swallowing Zeus.
Then Zeus led his brothers and sisters into battle against his father Kronos and the rest of the Titans. Zeus and his brothers and sisters won the battle and Zeus became the almighty king of the Greek gods.