Mariah Carey Star of Million Dollar Bat Mitzvah

Mariah Carey was the headliner of her manager's daughter's Bat Mitzvah ... but someone's got to tell her the meaning of "mazel tov!"

Mariah made a grand entrance into 1 OAK in WeHo Friday for Mishka Bulochnikov, the 13-year-old daughter of Stella.

The event was insane ... a 500 lb chocolate and red velvet cake, mini Kanye from Jeff Beacher's Madhouse, a Bentley golf cart which Mishka got as a present, a DJ from Power 105 ... but the thing is ... it cost hundreds of thousands of dollars to rent the club out on a Friday night.

Kids ... they're expensive.