Lenny Dykstra's Son: Sorry, Dad ... YOU'RE NOT A HALL OF FAMER

File this one under: Kid's Say The Darndest Things.

Lenny Dykstra's son Cutter must be a summa cum laude graduate of Keepin' It Real University ... and not even family is immune to his straight talk.

Cutter was leaving The Nice Guy in Hollywood with his wife -- Jamie-Lynn Sigler -- when we asked if he thought his dad should join Ken Griffey Jr. and Mike Piazza in Cooperstown.