Nigeria kidnappers free UK girl
Kidnappers kept Margaret in a hut for four days
Margaret Hill's parents, Mike and Oluchi Hill, said she was in good health but "covered in mosquito bites".
The kidnappers had threatened to kill the girl unless a ransom was paid or Mr Hill, a British oil worker, took her place. The family deny paying a ransom.
UK Foreign Secretary David Miliband said he was "delighted" at the release.
"I am grateful to the Nigerian authorities for all their help and I hope the perpetrators will be swiftly brought to justice," Mr Miliband said.
The toddler was taken to hospital for an examination before being reunited with her parents on Sunday night local time.
Margaret had been on her way to school in Port Harcourt on Thursday when gunmen smashed a window of the car she was being driven in and snatched her.
The car's driver was stabbed as he tried to protect the girl.
Mr Hill told the BBC that, despite having been held in a hut and bitten by mosquitoes, his daughter was "very active, not much worse for wear".
He said he would continue to live in Nigeria for the time being -but that he and his family would probably take a holiday.
"I live here and I work here and I've been in Africa since 1973," he said.
"Just because people get mad, they kidnap the children. It's very worrying but things have got to get better - they can't get worse.
"If it frightens everybody away, the whole country will be empty."
Mrs Hill, who is Nigerian, said the abductors had contacted her and demanded an unspecified ransom for Margaret's release.
The kidnappers had vowed to kill the girl unless her father, a British expatriate oil worker originally from Murton in County Durham, took her place.
The President of Nigeria, Umaru Yar'Adua, had called for the girl's release, and the region's main militant group - the Movement for the Emancipation of the Niger Delta (Mend) - had offered to help find her.
More than 200 hostages have been taken by armed groups in the last 18 months in Nigeria but none has ever been killed.
Margaret was the third child to be kidnapped in the last six weeks.The daughter of a Nigerian businessman and a local state legislator's daughter were both eventually released unharmed after ransom payments.