Beth Cole, from Croydon, said she was forced off the bus, issued with an £80 penalty notice and had her card taken off her.
"It was horrible," she said.
"I thought they were going to call the police and arrest me. It was a genuine mistake but they wouldn't listen to me.
"A woman on the bus even offered to make up the difference in the fare but the inspectors wouldn't let her.
"I was really upset and humiliated. I felt they acted like bullies."
Beth was using her mum's card after she had lost her own Zip card - which entitles children to free bus travel.
She reported the loss to Transport for London, and was told a replacement could take up to 10 days, so in the meantime she should use one of her parents' Oyster cards.
But she had not topped the card up with enough money to cover the £2.30 cost of her journey.
Her dad, Robert, said: "She was left in a very vulnerable position. I dread to think what might have happened if it had been someone less sensible or mature."
A TfL spokesman said: "If someone doesn't have a valid ticket for their journey they are liable for a penalty fare, and children must have their Zip cards with them at all times.
"People are taken off the bus so as not to hold up the service for other passengers, and they are issued with a permit so they can get home. Inspectors are completely professional and polite at all times."
Has your child ever been told to get off transport for not having the right fare?