Kim Zolciak's Ex Kroy Allegedly Dated OnlyFans Model ... Took Her on Vacay With Kids

'Real Housewives of Atlanta' alum Kim Zolciak's estranged husband, Kroy Biermann, recently dated an OnlyFans model named Bri and took her on a family getaway with his kids ... according to new legal docs from their divorce.

The alleged relationship is laid out in a letter written by the mother of a girlfriend of one of Kim and Kroy's sons. The letter, obtained by TMZ, was sent to the court-appointed guardian who makes custody recommendations in Kim and Kroy's divorce. 

In the letter, the woman says her daughter went on a trip with Kroy and his family over spring break. She says Kroy never mentioned his girlfriend, Bri, was going to be there … and she later learned Bri was an OnlyFans model. 

The concerned mother writes ... "This is a concern due to the apparent poor influence of his girlfriend. Although much of her sexual content has been removed from the internet, I did see the photos before their removal."

She continues ... "The woman is not someone who needs to be around teenage kids or giving advice on their behavior. My daughter knows she is not allowed to be around the girlfriend for an extended time."

The mother adds ... "I would assume Kroy would feel the same way about me taking one of his children on a trip without my family, if I had an OnlyFans male model as my guest."

She said Kroy even started screaming at her daughter on the trip and told her, "My son can't save you." The mother said removing the kids from Kim's care would be detrimental to them. 

In the letter, the mother also explained why she feels Kroy's comments that Kim was not a good mother were not true ... writing, "She is in contact with her children around the clock, monitoring their well-being, location and safety.”

Kim is using the email, along with an alleged text from Bri to Kim’s current boyfriend, Kyle Mowitz, about Kroy, as evidence in their bitter custody war. She's also claimed Kroy had loud sex their children overheard.