The Joplin Police Department arrested Simpson December 6 after receiving a call at a local hospital about an abused four-year-old.
The other person charged, Adam Duncan, 28, Joplin, was taken into custody Tuesday after a five-day hunt.
Duncan is also charged with two counts of felony child abuse.
Duncan's crimes are described in the probable cause statement, with the first instance allegedly taking place on or between May 6 and May 11.
While living with his girlfriend, Haley Simpson, Adam Duncan spanked Simpson's four-year-old son as punishment for failing to use the toilet.
During this time, Simpson and Duncan both spanked (the child) with a bare hand to (the child's) bare skin. (The child) suffered finger-shaped bruises to his buttocks and waistline from the spankings.
Further, on December 5, 2018, Adam Duncan committed the crime of abuse of a child. During this instance, Duncan physically struck (the child's) head with his hand multiple times causing serious bleeding to the right side of his head. (The child) also had serious bruising to the back of his hands and thighs caused by physical abuse.