Musa Ahmad, who works with the military against Boko Haram Islamists, said the incident happened at about 11:30 am (1030 GMT) in Banki, near the Cameroon border in Borno state.
“A girl of around 10 came by the military barracks and was trying to cross the road into the IDP (internally displaced persons) camp,” he told AFP.
“She was asked to stop by soldiers. But she ignored them. They threatened to shoot her if she didn’t stop. She obeyed and she was asked to lift up her hijab.
“She did and explosives were found to be strapped on her. Suddenly she pulled on the trigger and exploded.”
No-one else was injured, said Ahmad, who added it was the second attempt on the camp in recent weeks.
On January 18, a young woman and a teenage boy were shot when they refused to stop for a search and exploded just outside the camp, he added.
The vigilante suggested the attempted bombings were a way of Boko Haram “trying to hit back” after recent military operations against them in the area.
Banki, which is 133 kilometres (80 miles) by road southeast of the Borno state capital, Maiduguri, is currently home to thousands of IDPs.
In April last year, at least seven people were killed at the camp in a suicide bomb attack carried out by two women.
Boko Haram has frequently deployed young women and children as suicide bombers to target crowded places such as mosques, markets and bus stations.
On Tuesday, at least one person was killed when a suicide bomber exploded at a mosque at Dalori, on the outskirts of Maiduguri, during morning prayers, residents and the authorities said.
Shuaibu Abubakar said the blast happened at about 5:30 am (0430 GMT) as a suicide bomber tried to enter the place of worship.
“One of the worshippers, who was apparently on guard, grabbed him and the explosives went off, killing both of them. Worshippers were saved,” he said.
Tens of thousands of people displaced by the conflict are currently living at two camps set up at Dalori.
On January 30 last year, at least 85 people were killed when militant fighters stormed and torched Dalori, sending local residents fleeing into the bush.