The girl was paralysed from the waist down after her spine was crushed when she was raped, Reuters reported, and her 28-year-old uncle was suspected of the assault.
Bio said hundreds of cases of rape and sexual assault are reported each month in the West African nation against women, girls and babies as young as three months old.
"She may never walk again, and I want vengeance for what has happened", her grandmother said.
More than 8,500 assault cases were recorded last year - a rise of almost 4,000 on the figure from the previous year - in a country of 7.5 million people, the BBC reported.
Last year, a 56-year-old man who raped a six-year-old girl was sentenced to a year in prison.
"With immediate effect, sexual penetration of minors is punishable by life imprisonment", he said.
Around a third of cases involved the rape of a minor.
Police statistics cited by VOA state that reported cases of sexual and gender-based violence almost doubled past year to over 8,500, a third of which involved a minor.
Legally, rape is a criminal offence which carries a jail term of between five and 15 years, according to the 2012 sexual offences act - a law that Sierra Leone reworked and toughened specifically to combat sexual predators.
"Some of our families practise a culture of silence and indifference towards sexual violence, leaving victims even more traumatised", he told a crowd at the State House.
The government will engage communities and civil society in dialogue to end the scourge that is slowly wrecking the nation, the president said.
In December the first lady led a protest in the capital, Freetown, to raise awareness of gender-based violence. Only 12 years ago parliament passed its first gender equality laws in 46 years of independence, following lobbying by women's rights groups.
Authorities and security agencies have, however, been slow in implementing these policies and this has been blamed on lack of funding and other resources. Mrs Bio has been championing the latest wave of campaign against sexual violence through her "Hands Off Our Girls" campaign.
Fatmata Sorie, president of LAWYERS, an all-female group of barristers providing pro-bono legal services to vulnerable women and girls, said: "We still need to think about how services for survivors are not accessible, especially for the poor".
It had been made possible by the tireless voices of survivors and activists, she said.