Wearing a more modest outfit than her scantily clad adult rivals, Lira was seen shuffling along unhappily in a crowd of Brazilian photographers and cameramen, all but obscured to the 90,000 spectators in the purpose-built Sambodrome stadium.
Her mother told reporters the girl had cried and had to be picked up at the start of the show, but bravely carried on to finish.
Lira reportedly said dancing with all the attention "was hard" before she was whisked away from the lenses.
The incident deepened a sour note over the Carnival partying, which was already suffering one of its worst crime surges in years.
The bloody body of a Brazilian girl, aged around 10 years, was found late Sunday in bushes near her home in central Rio, the victim of an apparent rape-murder.
According to police, the girl's Carnival disguise had been half ripped off.
Also Sunday, an off-duty Brazilian policeman was arrested for shooting dead a fellow off-duty officer in a Carnival party, sparking panic among revelers in the club.
The G1 news website quoted investigators saying the two had a dispute and the officer who shot his colleague four times claimed he had acted in self-defense.
The two killings followed the shooting last Thursday of a Dutch tourist by a mugger. The 37-year-old Dutchman, who had just arrived in Rio with his wife to attend the Carnival, survived the attack but was in a serious condition in hospital.
With the parades continuing on until early Tuesday, and street parties going on for days after that, authorities are hoping the rest of Carnival will go on without other incidents.
Celebrities including pop icon Madonna, Paris Hilton, "300" star Gerard Butler, and Pussycat Dolls singer Nicole Scherzinger were in town to watch the spectacle.
Madonna even wanted to take part, leaving a VIP box she was sharing with her Brazilian toyboy boyfriend Jesus Luz and her children at one point to try to bust some samba moves in the middle of the parades.
But according to the Globo television network that exclusively broadcasts the event, she had to quickly retreat when a media and security scrum ensued.
Hilton, who watched from another private box with her American boyfriend, was expected to play up for the cameras herself later, as part of her contract to promote a Brazilian beer brand.
The two witnessed a flow of imaginative and colorful floats and an army of costumed dancers that each samba school deployed at a cost of up to four million dollars.
Most were historical or religious allegories, but a couple went overboard in embracing Hollywood imagery.
Thus there were also superhero riffs featuring Batmen on skis and Spidermen climbing under the lights. The undead buccaneers from the "Pirates of the Caribbean" movies also proved popular.
But the most surprising appearance -- warmly welcomed by the crowd -- was that of a Michael Jackson impersonator.
The actor had some of the late entertainer's moves down, and -- it appeared -- similar plastic surgery. Eyebrows were raised, though, by the fact that he shared the float with extraterrestrials.
A billboard on the back of the vehicle, next to an elongated E.T., showed a photo of the real singer, who died in June last year, with the words: "Michael lives in all the stars -- God bless him."
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