Sources with direct knowledge tell TMZ ... Kim just stopped by Richard J. Donovan Correctional Facility near San Diego to speak to a group of around 40 inmates -- including the Menendez brothers.

She was joined by her sister Khloe Kardashian and mom Kris Jenner ... as well as film producer Scott Budnick -- and, shockingly, actor Cooper Koch who plays Erik in the new Netflix biopic series, "Monsters: The Lyle and Erik Menendez Story."

We're told the superstar group went to talk about all the work they're doing on criminal justice reform ... as you know, Kim's super passionate about advocacy on the issue -- regularly talking to incarcerated people about various projects she's backing.

One of their major discussion points ... Greenspace, a reform strategy about beautifying prison yards to help inmates rehabilitate -- spearheaded by Lyle Menendez. Erik's also a huge part of the project.

As we told you ... Erik recently called BS on Murphy's new series -- blasting the show as a "naive and inaccurate" depiction of the killings that made worldwide headlines.

He said RM went out of his way to paint him and his brother in the worst light possible by leaving out the allegations of abuse against their father.

Of course, Erik and Lyle are serving life sentences for brutally murdering their parents, José and Mary Louise “Kitty” Menéndez, with a shotgun back in 1989.