The two even spoke to one another during the 90-minute event, which was held at a children's play center in East Sussex, England on Saturday, Britain's Hello! magazine reported Monday. About 15 guests attended, the magazine said.
McCartney reportedly took off in a car with Beatrice 10 minutes before Mills McCartney departed with leftover birthday cake in tow.
McCartney, 64, and Mills, 38, announced they were separating in May after four years of marriage. McCartney filed for divorce in July.
Their marital breakup has stirred a sensation not seen in Britain since Prince Charles and Princess Diana parted ways a decade ago, sparking immense rivalry in the British press for the best scoop.
Two weeks ago, the Daily Mail splashed its front page with what appeared to be legal papers drafted by Mills McCartney's lawyers against the former Beatle that alleged drug use, drunkenness, callousness about his wife's amputated leg, indifference to her pain, and even an assault with a broken wine glass.
Her lawyers said last week she is suing two newspapers over "false, damaging and immensely upsetting" stories surrounding her divorce from McCartney. She had been vilified in the media and was now being stalked by photographers, the firm said.
Associated Newspapers, the publisher of the Daily Mail and London's Evening Standard, said the stories about Mills McCartney in both papers "were obtained by proper methods and in accordance with good journalistic practice."