Elisabetta Grillo, 41, who also accused the celebrity couple of lying under oath, told jurors Ms Lawson permitted the teenagers to smoke cannabis as she gave evidence in her fraud trial.
The defendant, who along with her sister Francesca, 35, is accused of spending £685,000 on credit cards belonging to the celebrity couple to buy designer goods and luxury holidays for herself, made the allegations as she spent a second day in the witness box at Isleworth Crown Court in London.
Elisabetta, 41, was asked by prosecutor Jane Carpenter to explain a duty-free transaction for £69.71 in New York in June 2010.
"It was cigarettes for the children. I bought them and Nigella allowed me to buy them," Ms Grillo said.
Asked by Ms Carpenter: "What on earth did you think you were doing, buying cigarettes for underage children?"
Elisabetta said: "Well, if Nigella Lawson let the children smoke weed..."
The defendant tailed off as Judge Robin Johnson interrupted the exchange.
Elisabetta told the court that Mr Saatchi only discovered his former wife's alleged drug habit on the day he was photographed with his hands around her throat outside Scott's restaurant in Mayfair, central London.
Jurors were told Grillo had not originally planned to use her former boss's alleged drug taking in her defence in an effort to protect the TV cook.
An original defence case statement for Elisabetta Grillo from August did not include allegations of Ms Lawson's drug use because she did not want them raised in court out of a "remnant of sympathy" for her, jurors heard.
But an extra statement added in November did include the claims.
Jurors previously heard that the defendants allege that Ms Lawson, who divorced Mr Saatchi earlier this year, regularly snorted cocaine and smoked cannabis during her 10-year marriage to the multi-millionaire.
But last week, Ms Lawson claimed she had only taken cocaine with her late husband John Diamond when he found out he had terminal cancer, and on another occasion in July 2010 during her troubled marriage to Mr Saatchi.
The Grillos, of Kensington Gardens Square, Bayswater, west London, each deny a single count of committing fraud by using a company credit card for personal gain between January 1 2008 and December 31 last year.
The trial was adjourned to 10am on Monday.