Sex crimes Sgt. Paul Mumford said detectives think Johnson might have abused other children because he often baby-sat in the Port Tampa area.
Johnson's mother and brother, however, said he has a limited mental capacity. They said he has epileptic seizures and cannot read or sign his name.
"I don't believe he's accountable for it," said Johnson's mother, Nancy, 51. "I believe he was persuaded by the other kids who have a better brain than he does."
Edward Johnson, 27, added that his brother can be impressionable.
"He's got the brain of the same people he's hanging out with," meaning children, he said. "If he really did do these things, he needs to go somewhere and get help."
Police spokeswoman Laura McElroy said detectives talked to Charles Johnson and his family at length about his ability to understand right from wrong.
"We believe that he's competent and knows the difference between right from wrong," McElroy said. "He was aware that what he did was wrong."
McElroy said police also noted that Charles Johnson is a high-school graduate.
His mother said he graduated from Jefferson High School in 2002 as part of the Trainable Mentally Handicapped program. The Hillsborough County School District could not confirm that because of privacy issues.
However, Hillsborough County School District spokeswoman Linda Cobbe said that Johnson sporadically helped out in the kitchen at Jefferson as an unofficial volunteer, which meant he did not undergo a background check.
"It's not something we encourage," she said.
Public records show Charles Johnson has no prior criminal record in Florida.
Nancy Johnson said she appeared before Circuit Judge Raul Palomino on Monday with her son regarding a protective order the 9-year-old's mother had sought against her son. Court records show that protective order was dismissed.
The mother wrote in court papers that Johnson was seen reaching down her son's pants at the apartment complex.