BEING a child genius may not lead to a glittering career, according to a study published this weekend. The report quotes research in San Francisco which found that 1,500 children with IQs of at least 135 had not been noticeably more successful than those randomly selected from the same social background. Another study of 200 pupils from the Hunter School for the Gifted in New York (average IQ 157) showed that none had reached the top of their profession. Of 11 figures identified as 'world changers' by a Harvard professor, only one - Pablo Picasso - had shown his stature by the time he was 20. Others, like Albert Einstein and Sir Winston Churchill, did badly at school. As if that was not enough of a discouragement for youthful super-achievers, research in Britain has concluded that children labelled gifted were often less happy than more average young people.