The Saving Private Ryan star is said to have touched the child's genitals, but her parents opted against pressing charges and he was later allowed back onto the set.
Sizemore's agent Stephen Rice told THR that the actor had declined to comment on the allegation.
Allegedly spurred by the recent unprecedented conversation in Hollywood surrounding sexual harassment and assault, following the bombshell allegations against Harvey Weinstein, the actress in question reportedly has hired a lawyer and is looking into what can be done.
In the story posted online on Monday, November 13, a dozen cast and crew confirmed the incident to THR, saying that it took place towards the end of filming Born Killers, which is about two brothers on a crime spree.
The actor, who had a history of drug use and charges of battery against women, was dropped by his management firm and agency shortly afterward.
Sizemore's other film roles include Born on the Fourth of July, True Romance, Natural Born Killers, Heat, Black Hawk Down and Pearl Harbor. This is when he allegedly "rubbed his finger against her vagina" or 'inserted it inside'.
Robyn Adamson, who portrayed Sizemore's wife in the pictures, offered a devastating account of what she witnessed.
"At one point her eyes got just huge, like she could've vomited", Adamson said. "I was watching her", she said. She added that afterward, "the young girl had trouble taking direction".
The movie's producers Jai Stefan, Michael Manshel and Gus Spoliansky also reveal to THR they reviewed the photographs from the portrait session but found them to be inconclusive evidence and encouraged the parents to take legal action if they so felt necessary, but Stefan recalls the parents "not wanting the little girl being taken off the movie. We're talking about consistent behavior, just being "Tom Sizemore" on set every day", Maufas stated.
At the time the film was being shot, under the name "Piggy Banks", Sizemore had recently been convicted of harassing and physically abusing his ex-girlfriend Heidi Fleiss. He has publicly battled drug abuse since he was a teen, and was charged with with possession of methamphetamine in 2007.
The very next day, the casting director, Catrine McGregor, received a call from the young girl's mother who told her what had happened.
The story quickly filtered down through the film crew, who according to Production Assistant Roi Maufas, were ready to go after Sizemore themselves. "The little girl said what she said and we all thought, "That (expletive) sleazebag.' There was never any doubt". "He was this guy who was already known for making inappropriate comments, being drunk, being high".