Tannine Montgomery, 30, said she was stopped at Stansted on Friday after returning from the Netherlands, where she obtains the medicine using a private prescription written by her UK doctor.
Indie-Rose Clarry, who has Dravet syndrome, a form of epilepsy, suffered severe seizures before taking the oils for the past 14 months, her parents said.
"It's a really frightening place to be watching her seizure and knowing there wasn't anything controlling that," Ms Montgomery said.
She added that Indie's condition has dramatically improved with regular doses of medicinal cannabis.
It is now legal in the UK but Indie's parents cannot get it on the NHS and importing it costs £400 a bottle.
For the past four months Indie-Rose's dad has been bringing it back from Amsterdam at a fraction of the cost.
He told ITV News: "Even though I have a written prescription by a paediatric oncologist from London and I produced that to show them and they said regardless of any paperwork or anything like that, if a product comes up positive on their test it will be seized."
Indie-Rose's parents gave ITV News permission to use a family video of their daughter having a seizure to show how bad things were before using cannabis oil.
Warning: this article may contain distressing images
Both Ant and Tannine desperately want the medication available on the NHS.
Campaigners have said that the legalities of importing cannabis oil privately is a grey area.
But on Saturday the Government told ITV News it is illegal to import controlled drugs without a Home Office license and Border Force has a duty to enforce the law.
Peter Caroll, from the campaign group 'End Our Pain', told ITV News: "The medical establishment, the trusts and the doctors seem very nervous, very worried about prescribing this medicine because they fear it might have long-term effects on the child's brain but our point here is that we are talking about children who are desperately ill."