Suzanne Fairbairn, 53, who calls herself "Lady Sam", said the money would go to the nine-year-old boy, the product of her husband's affair with an Australian teacher.
The same amount will be given to each of his three daughters from his first marriage - despite his decision to cut them out of his will hours before his death in February.
The money will be paid out from the sale of the MP's beloved home Fordell Castle, in Fife. The 13th-century castle, which Sir Nicholas always said he bought for pounds 100 and rebuilt, is being sold to a Scottish businessman for more than pounds 500,000.
Lady Fairbairn, who married the MP in 1983, is selling the castle to settle expenses and because she wants to make a fresh start in the south of England. She said that although her husband would have opposed the sale she feels there is no option. The castle is too expensive to run and there are memories she now wants to leave behind.
After Sir Nicholas's death she discovered that he had fathered a son during their marriage. The child's mother, "Paddy" Mullen, wrote to the family shortly after Sir Nicholas died saying she wanted her son, Edward, to visit Fordell and meet his three half-sisters.
Lady Fairbairn has since found photographs of the child among Sir Nicholas's papers along with a Christmas card, signed by the boy. She said: "I find it, to be frank, that he should father a son so soon after we were married, really rather bad judgement on both their parts. I did not expect fidelity but I thought that was really not very good. I thought the timing was very poor. She had been coming here since 1978, it had been going on that long. It makes the mourning easier . . . I still love him. I just feel that it was completely ill-judged."
Lady Fairbairn denies there was a rift with the daughters which resulted him cutting them out of his will. A codicil, which he added hours before his death, revoked legacies of pounds 10,000 to each of them. "I knew he was doing it. There wasn't any money to give them. There will be of course once Fordell is sold. They will get their money."Reuse content