Firstly, there is another reviewer here (or on the blue one, I'm not sure) who states that the strap is coming off easily due to it only being a Velcro attachment at the adult end. I think your wearing it incorrectly. The adult end of the strap needs to be guided through the plastic loop before securing the Velcro. Then it will not come off. I know as I checked this immediately upon the products arrival (due to your review) and after reading the instruction manual all was well.
Anyway, this is a great product. Our daughter outgrew the torso harness thing so this was the next step. It's just the right length and also features a little elastic shock absorber in the middle to prevent your toddler hurting their wrist if they yank/fall, etc.
Highly recommended