Isabel Mercedes Celis' parents, Sergio and Becky Celis, wore T-shirts with "Bring Isa Home" on the front and a photo of the brown-haired child with hazel eyes on the back, the Arizona Daily Star reported.
"We will do anything for her," Sergio Celis said at the Wednesday news conference.
He made a plea to "the person or persons who has Isabel" to "tell us your demands."
Tucson Police Chief Roberto Villasenor said authorities have not received a ransom note but said a demand could come.
Police said they don't believe Isabel has been taken out of the state and are operating on the assumption she is alive, the Star said.
Isabel's family told police they last saw her when she went to bed at 11 p.m. Friday and she wasn't there when her father went to wake her at 8 a.m. Saturday.
Villasenor said Wednesday he didn't believe Isabel walked away from her home.
The police chief said there had been more than 300 leads in the case.
Officers are reviewing surveillance video from cameras near the home, Villasenor said, but he wouldn't say whether it has yielded any clues.
More than $33,000 has been pledged in reward money in the case.
Authorities urge anyone with information about Isabel's disappearance or whereabouts to call 520-882-7463.