Kelsey Grammer and Kayte Pull a BRITNEY with Baby -- NO CAR SEAT!

Kelsey Grammer's 4-month-old baby doesn't just party at the Playboy mansion ... she also dangerously rides in vehicles WITHOUT A CAR SEAT ... and it was all caught on tape.

Kelsey and his wife Kayte arrived at LAX yesterday with their adorable (and fragile) baby daughter Faith ... when they were met by a waiting chauffeured SUV.

But as Kayte was about to enter the car with the kid, she realized there was no car seat in the SUV ... but she got in the car with the baby anyway.

Kayte seatbelted herself in the car, and with the baby cradled against her chest, the car took off ... a clear violation of the California car seat law.

We're assuming the baby made it home unfazed ... but according to various safety organizations, it's a STUPID risk to transport your newborn without a proper child safety seat ... considering car seats can significantly reduce the risk of injury in an accident.

Of course, Britney Spears took all sorts of heat when she drove around with her baby in her LAP several years ago.

Kelsey's rep, Stan Rosenfield, insists there was a car seat inside the vehicle ... though there's no evidence of that in this video.