"Today has been a long day finding out my [daughter's] brother passed away and knowing that she never even got to meet him."
Sources connected to the situation tell us ... the mother of AP's 5th child first met the NFL star while serving him as a VIP waitress at a place called Seven -- a steakhouse, sushi joint and ultralounge all wrapped in one.
It's unclear how involved Adrian has been in the child's life -- but the mother recently posted a photo showing the child decked out in Minnesota Vikings gear, so we're guessing they're cool.
Peterson has fathered 4 other children -- an 8-year-old daughter, a 4-year-old son, a 2-year-old son and another son, who died just months after AP learned he was the father.
We called AP & the child's mother for comment -- so far, no word back.