28-year-old Kiarra Jones is facing 2 counts of crimes against at-risk children in Denver after she got busted on April 4 ... when a little boy's parents reported he came home with bruises one day -- this after months of inexplicable injuries that would surface on his body.

Cops say they got their hands on the bus surveillance footage -- which has been making the rounds online ... and it's best to watch at your own discretion, as it shows a brutal attack.

Jones is sitting next to the boy, and appears to elbow him in the stomach, smack him in the face, and if that wasn't enough, even stomp on his foot. What makes it all the more heart-wrenching is the helpless kid trying to distract himself by looking out of the window.

The boy's mother, Jessica Vestal, says she released the footage Tuesday to make sure something like this doesn't ever happen again. Since then, Jones has been booked. Another mom, Brittany Yarborough, posted photos of injuries her own son sustained, including a red and swollen ear ... and says she believes Jones might be responsible for that as well.

Cops say they also found footage from 2 other bus rides dating all the way back to mid-February ... where Jones appears to be harming other autistic kids. Police are investigating.

Goes without saying, Jones was immediately given the boot from her position after her arrest. She was released from jail after posting bail. Her next court date is scheduled for May 3.