She was arrested by French authorities at the northwestern port of Dieppe before they could board the vessel to Newhaven on Britain's south coast.
The mother-of-one, who works in a London suburb as a waitress and occasional escort girl, had arrived in northern France to visit "The Jungle" migrants' camp in Calais.
According to the French authorities she accepted 500 pounds (650 euros) from an Iraqi migrant at the Calais camp to smuggle the 15-year-old across.
She accepted the task, she said, for "humanitarian reasons".
She was originally sentenced in November to one year in jail.
On appeal at a court in the northeast town of Rouen that sentence was eased to three months in jail.
Including time already served in custody she should be freed on February 12.
The appeal court also banned her from entering France for five years.
Last week French justice showed more clemency to another British voluntary worker, former soldier Robert Lawrie.
He was only fined 1,000 euros for attempting to take a four-year-old Afghan girl out of the Calais camp in order to drive her to Britain.
Unlike his compatriot he received no money and his act was deemed to be unpremeditated.
The camp, known as ‘The Jungle’ has around 4,000 inhabitants, mainly from Syria, Afghanistan and Iraq but also Sudan and Eritrea.