Claus von Bu low's stepchildren appealed in New York yesterday to his former mistress to come forward to repeat her damaging testimony at the socialite's retrial on charges of twice attempting to kill his wife.

Annie-Laurie (Ala) Kneissl and Alexander von Auersperg addressed their statement to former soap opera actress Alexandra Isles, 39, who is believed to be staying in Europe to avoid testifying at von Bu low's retrial in Providence, R.I., Superior Court.

Kneissl referred to Isles' background: "There's my mother's life in this. My mother is flesh and blood, and this is not a soap opera."

Isles testified at von Bu low's 1982 trial that she set a deadline for him to leave Martha (Sunny) von Bu low and marry her. Prosecutors say that deadline was shortly before Sunny von Bu low's first coma in 1979.

Von Bu low, 53, has remained in a coma since the second incident in December 1980. Prosecutors say her husband injected her with insulin, bringing on the comas.

The stepchildren said they came forward because of the urgency of the situation. Judge Corinne T. Grande gave prosecutors until Tuesday to produce Isles.

The state alleges von Bu low, 58, tried to kill his wife so he could inherit $14 million from her estate and marry Isles. His defense alleges his wife's coma was brought on by abuse of drugs, alcohol and sweets.

He faces up to 40 years in prison if convicted on the two counts of attempted murder. His 1982 convictions on the same charges were overturned by the Rhode Island Supreme Court. Barnard Backs Euthanasia

Dr. Christiaan Barnard, the South African heart-transplant pioneer and an advocate of euthanasia, has given his support to a British couple who killed their quadriplegic son with an overdose of drugs.

"I fail to see why my profession is not already living up to its responsibilities," Barnard wrote in an article in London's Mail newspaper on Sunday. He criticized doctors who saved the life of Robert Houghton, 22, after he was paralyzed in a 1980 road accident, saying they "should have allowed Robert to die."

Barnard's article said he had given his 80-year-old mother "dignity in death" by ordering no medical treatment after she suffered a stroke, "and I have done that for my patients in the past."

Houghton's parents, Norman and Janet Houghton, were charged with murder and found guilty of manslaughter by a jury May 14 for killing their son in October 1984. The judge freed them, however, saying they should not be punished further.

Barnard, who is visiting Britain, said he had met with Janet Houghton and he quoted her as saying her son wanted to die and "of course I hated giving him those pills. But who else would have done it?"

He wrote, "As a parent I cannot conceive of a worse decision. As a doctor, I am ashamed they had to make it." End Notes

A boutique assistant refused to exchange a pair of trousers for Princess Diana unless she produced a receipt, a British newspaper reported yesterday. The Mirror paper said amazed shoppers watched as the store clerk demanded proof of purchase from the future queen of England for the $26 trousers, which were too large. After the manager of the Benetton shop, in London's fashionable Kensington district, settled the dispute, another assistant told Diana she must produce a receipt in the future . . .

Cartoonist Charles M. Schulz, 62, says that when he dies, so will his 35-year-old comic strip, "Peanuts." "That's what my children want," Schulz said during a recent visit to Hallmark Cards Inc. in Kansas City. "They said, 'No, nobody else is going to draw Dad's strip.' I don't care what happens. It just makes me feel good about my children," said the cartoonist. . .

The National Park Service says it will no longer interfere with nude sunbathers who show up at beaches in 10 national seashore areas. "We do not condone nude sunbathing," George Berklacy of the Park Service said last week. "But we're going to treat all bathers the same." There is no federal law against beach nudity.