Fish scales, no matter how accurate or inaccurate, are only good for measuring static weights. The braking force required to stop a vehicle varies directly with its weight and speed, and the proportion isn't linear - its exponential. It shouldn't take a genius to figure out that the speeds attained by a seven year old wouldn't approach that of an adult, nor would the weight, nor would the braking forces. Its pretty easy to stop a spinning wheel on a bike in a stand using any brake. Thats absolutely no indication of what to expect in the street with a rider.