EXCLUSIVE

At least one of Octomom's kids watched her masturbation video ... according to one of Octo's former nannies.

TMZ has obtained correspondence between nanny Gina B and the Orange County Dept. of Children and Family Services ... in which she claims on June 28, Nadya ran to her crying, claiming she found her oldest son watching porn on his phone.

The nanny says the 11-year-old had found the website that posted his mom's masturbation video and decided to sneak a peek. The nanny says Octo handed her the phone and asked her to block the site, but when the nanny said she needed a passcode, Octo told her to forget it.