no this is not a sign of bruising but it may be a sign of contact reaction. since the redness and swelling has occurred in both hands at the same time there is a possibility that he picked up more than just a sand bag. if he picked up or handled cement this is an entirely different problem. cement does cause the hands to become red and irritated. did he put his handprint into some fresh cement? i think you need to do a little more investigation of a nine-year old boy's day.
my son 8 years old has swollen and red hands. he was picking up a sand bag yesterday is this a sign of brusing?