A second reporter and the girl's mother were shot and injured by the same gunman, who is also suspected of the other homicide, police say.
The two Spectrum News 13 journalists were covering the killing of a woman in the area that morning when the teenage suspect returned, police say.
It is unclear if they were targeted.
The suspect was armed when he was arrested and was not co-operating with police, said investigators.
None of the three people killed in Wednesday's two attacks in Pine Hills, a suburb west of Orlando, has yet been identified.
In a news conference, Orange County Sheriff John Mina said the journalists were "in or near their vehicle", which he said did not look like a TV station's official vehicle, when they were attacked at around 16:00 local time (22:00 GMT).
He said the journalists had been reporting on a shooting that took place earlier in the day at around 11:00 local time, which saw a woman in her 20s fatally shot inside a car, when the suspect returned to the crime scene and opened fire.
After attacking the journalists, the alleged gunman - Keith Moses, 19 - went into a nearby home and shot the girl and her mother, the sheriff said.
The mother was in hospital in a critical condition, he added.
Other journalists nearby helped provide first aid to the victims, according to local reporters.
Spectrum 13 continued live coverage after the death of their reporter was announced.
Greg Angel, a news presenter for the station, said the injured journalist had "been able to speak with investigators and colleagues".
The suspect, Mr Mina said, "has a lengthy criminal history, to include gun charges, aggravated battery and assault with a deadly weapon, burglary and grand theft charges".
He described him as an "acquaintance" of the woman shot in the morning, "but as far as we know, he had no connection to the reporters and no connection to the mother and the nine-year-old".
Asked about the possibility that the gunman purposefully targeted the reporters, Mr Mina said "it's something we'll be taking a look at".
He added that it was also possible the suspect mistook the journalists for police.
A reporter for Orlando TV station WESH 2 reported that she and her camera operator had left the crime scene only moments before the shooting.
"We got a gut feeling" and decided to leave for their own safety, said Senait Gebregiorgis.
Charter Communications, the company that owns the TV station, released a statement calling the attack "a terrible tragedy for the Orlando community".
"We are deeply saddened by the loss of our colleague and the other lives senselessly taken today," the company said.
White House Press Secretary Karine Jean-Pierre tweeted condolences, saying: "Our hearts go out to the family of the journalist killed today and the crew member injured in Orange County, Florida, as well as the whole Spectrum News team."
There were 40 journalists killed in 2022, according to the Committee to Protect Journalists. Only one of those killed was in the United States.