HYDE PARK (CBS) — Parents are calling for more bus monitors after allegations surfaced that a six-year-old Boston Renaissance Charter School student molested other kids on their school bus earlier this month.

Mom Kiara Grinkley is outraged that she found out about the alleged sexual abuse through a fellow parent’s Facebook post several weeks after the incident.

“If the kids refuse,” a family friend of one victim wrote on Facebook, “[the six-year-old] beats on them and threatens them, saying he’s going to bring a gun to school to shoot them and their parents.”

It wasn’t until weeks after the alleged incident was reported to police that the school officials informed parents through a robo-call and a post on the school’s website.

Grinkley doesn’t even know whether or not her four-year-old son was on the bus in question, because no one will tell her.

“It’s definitely concerning to not have any clue if he was exposed visually, or exposed to even the threats,” Grinkley said. “We cannot find out because the child’s so young, the kid has a lot of protection.”

Boston Police said they were notified about the incident on November 3rd, but “Due to the ages of the parties involved there can be no criminal investigation.” Instead, detectives referred the incident to a social service agency that provides family counseling.

On their website, Boston Renaissance said, “We have taken this matter very seriously and have taken appropriate measures to ensure student safety.”

Grinkley is joining other parents and community activists in demanding bus monitors be placed on all school buses in the city for all students–whether they go to public schools or charter schools.

“Us as parents can call City Hall and advocate for our own children to make sure that there are monitors on these school buses,” said Grinkley. “Since the 1980s, because of budget cuts, monitors have been taken off of the school buses. Therefore, our children have been unprotected.”

Boston Public Schools said there were no monitors on the bus at the time of the alleged incidents. They said eight of Renaissance Charter School’s 21 bus routes have monitors on them.

“Our children are not safe on any school bus that does not have a monitor,” said advocate Mary Franklin.

District-wide, BPS said 37 percent of bus trips have monitors on them–the ones where attendants are required for special needs students. They said schools can request more monitors “on a case-by-case basis.”

Renaissance Charter School is not a part of the Boston Public Schools district, but rather is a Commonwealth Charter School. Boston Public Schools does, however, provide bus transportation.

BPS said they found out about this incident early Monday and began an internal review.  They told WBZ-TV in a statement they “immediately provided resources to help assist students who ride the bus and their families.”

“The safety and wellbeing of our students, and those we transport, are always a top priority of the BPS,” the statement read.

Mayor Walsh told WBZ-TV’s Beth Germano Tuesday that there is a cost issue with adding so many bus monitors, but said he would look into it.

“The issue is the cost, and making sure, how do we do it appropriately?” said Walsh.

The Department of Children and Families is also involved in the investigation.

WBZ NewsRadio 1030’s Kim Tunnicliffe reports