Monica and Marco Lira had joked that, one day, their third-grade daughter would dance her heart out as a drum corps queen in Rio’s annual Carnaval samba competition, which is seen by tens of thousands of paradegoers and millions more television viewers.

But then Mr. Lira, the president of one of 12 competing samba troupes, decided to go ahead this year, and put his little Julia at the head of an extravaganza of dancers, floats and drummers, in a sexually charged position usually reserved for bombshell actresses and models.

“My wife said, ‘You are going to create a problem for yourself, she is 7 years old,’ ” Mr. Lira said in an interview late Thursday. “I said, ‘What problem?’ ”

Rival samba groups scoffed, saying Julia was too young to provide the “sensual” inspiration that leads to winning performances. But child protection advocates took the case more seriously, accusing the samba troupe of child exploitation. Reportedly, it took a judge’s ruling to clear Julia’s path, putting a gap-toothed, pint-sized dancing queen at the head of her 3,200-strong samba school in the hotly disputed samba competition at Rio’s Sambodromo.