Simon Cowell -- I've Got This Baby Thing All Backward

Simon Cowell flashed his new daddy skills -- in front of all the paparazzi -- and FAILED MISERABLY at the dreaded car seat test.

Simon and Lauren Silverman took their newborn Eric to the doctor Friday for a checkup. Lauren said the baby's doing great -- and then it was Simon's time to shine ... or not.

Watch what happens when he attempts to load the kid into the car. If we're being honest ... Papa Cowell made a big no-no.