Heh heh, looks like it is in 'fashion' to use backpack leash on kids in Tokyo 
I did it when my DS was 4 yrs old and when we were in Tokyo. I didn't need to do it with my first born as she is not as adventurous as my DS.
He didn't mind it, and certainly is in no way 'traumastised'by it.
However, we didn't use it for long as DS figured out how to wriggle himself off the harness pretty soon...
SPI:I use the backpack leash on my 4 yrs old on recent trip to Tokyo Disneyland. Its is very useful and the backpack can carry a small bottle of drinks so its very convenient when she need to drink, I dont have to keep on digging into my bagpack. When she fall asleep in the bus or chair, it works as a pollow, neck support or even a chair cushion (Just nice in size). It also allow her to move around when I need to do some shopping. She loves her backpack so wheneven we at crowded place she will ask to leash her.