Making their first appearance in the tournament since their run to the semi-finals in 2005, Al Ittihad dominated the game at their home ground to set up a clash against African champions Al Ahly at the same venue on Friday.
The winners will face Copa Libertadores champions Fluminense in the semi-finals.
The hosts went ahead before the half-hour mark as Romarinho's shot from the edge of the box bounced off an opposing defender and into the net past keeper Conor Tracey.
France midfielder Kante doubled the lead five minutes later with a superb strike from a loose ball in the box before compatriot Benzema added a third in the 40th minute to become the first player to score in four different Club World Cups.
CONCACAF Champions League winners Leon face Asian champions Urawa Red Diamonds in their second-round tie on Friday. The victor in that fixture will meet Manchester City.
