W HILE it is generally accepted that children living in Hongkong lead stress-ridden lives, Whimsy - a mini play-land offering amusement by way of compensation to scowling kids forced to roam shopping centres with mum as she checks out fashion boutiques - appears to be going ridiculously over the top. A sign at one of the rides at its Cityplaza funfair informs the teenies and tinies lined up that they may not go on if they are suffering from ''heart trouble . . . high blood pressure . . .'' etc. Incidentally, when we called the Cityplaza management office in Taikoo Shing to check this out, a taped message responded, saying: ''Thank you for telephoning the Cityplaza management office during our lunch break . . .'' Bon appetite then - but do watch the cholesterol.