The boy's body was found Monday inside the van in the West Memphis day care's parking lot. He had been picked up at home before 7 a.m., but police said he never got off the van.
The van's driver 42-year-old Felicia Ann Phillips and three other employees, Pamela Lavette Robinson, 43, Kendra Washington, 40, and 43-year-old Wanda Taylor were fired on Wednesday
and have been charged with manslaughter.
Crittenden County Jail records show the women were in custody on Friday but have since been released. They are expected in court on Monday.
West Memphis Police Captain Joe Baker said the women avoided several protocol steps that could have prevented the incident.
"The things they did to bypass protocol were negligent, reckless and against protocol," Baker told CNN.
The women found the boy dead inside when they came to load children in the van to go home after 3 p.m., police said. The boy was still sitting on his booster seat and had removed his socks, shoes and shirt, Baker said.
He said the van doors were heavy and the sliding door was childproof so there was no way the boy could have opened the door to exit. When police arrived to the parking lot on Monday, the temperature was 91 degrees.
"They need to explain why and how they let this happen to my child," the boy's mother Ashley Smith told CNN affiliate WMC
Ascent Children's Health Services CEO Dan Sullivan said the staff did not follow company policies and procedures. Had they followed protocol "this tragedy would not have occurred," he said in a statement.
Taylor checked the boy off a list used to account for all the children traveling on the van without ever seeing him, police said.
She was covering for someone else who wasn't there that day, Baker said.
Phillips' responsibilities as the driver also included unloading the children and Robinson's job was to assist her.
Washington, called a "director designated van safety inspector," was responsible for checking if the van was empty after kids were unloaded, but police say she did not perform her job.
The boy had been going to that day care center since he was one years old. He had two heart surgeries in the past and was at the facility for development issues, CNN affiliate WMC reported.
The facility provides services "for children with complex health needs in addition to day care," according to its website.