You're never too young to party in Hollywood ... so says Kelsey Grammer, who took his 3-month-old baby to the Playboy Mansion for a Halloween bash and partied 'til the wee hours of the morning.
A party-goer tells us ... Kelsey was whooping it up at a table next to Paris Hilton just after midnight. Baby Faith was tucked into her bassinet as the music blared.
A Playboy Playmate with curious maternal instincts tweeted, "Why the fk does Kelsey Grammer have a newborn baby at the Mansion party!?!?!?"
We called Kelsey's rep, so far no comment.