Amazon have a Baby Toddler Safety Harness backpack with reins on sale for £3.17.
Before I had kids I absolutely hated people who put reins on their toddlers, but when my walking 8 month old toddled away from me in Paris, while I was looking at the map next to the Arc de Triomf, I quickly changed my mind. She was so fast, and I think I aged a lifetime in the seconds it too to find her again. We returned home and I dug her backpack with reins out of the back of the cupboard where I'd flung it muttering disapproving comments.
As it's panned out, we have both been glad for them. We really only used them till her mental ability caught up with her physical, and she was able to understand dangers, and understand staying by me, but she used to put the handle part in my hand and put the backpack on herself when she wanted to go for a walk! Yes, yes, a little like a puppy - but a safe and protected and happy puppy. Judge me if you like, but we're happy!
This is a small backpack, just big enough for a nappy and some wipes, or maybe for a little snack. The strap swivels to prevent tangling, and the sliding torso strap keeps the harness in the correct position.
It has a zippered back pocket and a removable tether strap. The bag is machine washable, and the straps adjust to fit the child. The strap is 32", so you're never really going to be out of arm's reach.
Thanks to jackthenippa at HUKD