A MAINLAND mother whose two-year-old son is suffering from cancer has been given permanent residency in Hong Kong. But for Yeung Wan Lai-hung it was a double celebration last week. Her son Ka-ho, who is suffering from a rare form of cancer of the testicles and has spent most of his young life in Tuen Mun Hospital, was allowed home for a short period last Thursday. Yesterday, Mrs Yeung, who collects her Hong Kong identity card next month, thanked the Sunday Morning Post for highlighting her plight. ''The public is most supportive; so is the Hong Kong Government and Ka-ho's doctor. If not for their support, I would have been deported to China,'' she said. Mrs Yeung was arrested in August - one day after giving birth to her third child - for overstaying her visa. After her arrest, she lived in fear Ka-ho, her eldest child, would eventually be left on his own in hospital while his father, Yeung Tak-ming, a Hong Kong resident, juggled his duties of looking after the boy and working in a factory to support his family of five. ''Our problem was while we knew Ka-ho needed 24-hour nursing, and the love and care of somebody dear to him, we could not give him that unless we had the help of the Hong Kong Government. ''I think the Government has made the right decision to let me stay. It has been most kind.'' Now, not only has Mrs Yeung's immigration status been solved, but so has the fate of her youngest child. The Tuen Mun Hospital's social worker, realising Mrs Yeung would not be able to tend to three children - one of them very ill - arranged for the newborn to go to a child-care centre. Mrs Yeung's other child is in China, being looked after by relatives.