According to the lawsuit, obtained by TMZ, Jessica Perry says she bought a 'Potter' replica wand with a light-up pen function from a store and gave it to her kids ... only for it to end up impaling her 3-year-old's eye.

Jessica claims her older son was waving the wand around -- like wizards do in the movies and books -- when the metal writing portion of the pen ejected from the wand, flew across the room and pierced her younger son's eye.

In the docs, she says the pen ruptured his eyeball, nearly leaving him blind and forcing him to undergo a pair of eye surgeries.

She alleges the wand was defective, and Warner Bros. should have known about the risks ... because she says there's at least one online review describing a similar incident.

In the suit, Jessica claims her son had to use painful eyedrops which made him scream in agony ... and she says the eyepatch he had to wear frightened him so much, he could only sleep with his bedroom lights on.

She also says her little boy now has to guard against injuring his retina again ... which means for the rest of his life he can't play sports, roughhouse or have a normal life.

She's suing WB for several million dollars -- $8 mil in compensatory damages for her son's injuries and emotional distress, plus punitive damages.