The child endangerment trial is set for March 18.
"Information led to the discovery of Jacob Kimbley's body from the septic tank," officials said, adding the lid was closed on the tank when the body was found.
Smith County District Attorney Matt Bingham said in August he could not confirm the lid was on or off of the tank.
"Whether the lid was on or off the septic tank is very relevant to the case and is one very important factual piece of evidence," he said. The children are now in Child Protective Services custody.
Officials said the roach infestation was so severe that roaches were falling from the ceiling onto officers as they searched for evidence in the home on a search warrant obtained Wednesday.
According to the court documents, David Kimbley was arrested in 2003 for driving while intoxicated, but no criminal history was found on his wife. The same documents also state that the Kimbleys refused to allow their remaining children to be interviewed by child psychologists.
In addition to the investigation of his parents, the child's grandfather, Leo Harber, was arrested on Aug. 22 for violating probation as a registered sex offender by living next to children at the location.
Bingham said the FBI, Smith County Sheriff's Office, Texas Rangers and Department of Public Safety along with his office and CPS are investigating the case.
The child's body was sent to Southwest Institute of Forensic Sciences in Dallas for an autopsy. The results were inconclusive.
Officials at the scene said the body appeared to have been submerged for quite some time.