I have one of these bad boys https://www.bicyclesonline.com.au/pacific-trailer-double and a fat labrador. I want to buy or easily fashion a harness so that the fat lab can pull the carrier with kids in it. Daycare is 500m away, and I'd rather have the dog pull the 2yr old than me carry him.
Web searches result in dog carriers or converting carriers to carry dogs. I have found 2 bike leashes, but they don't look like they'd support the carrier very well. Before anyone jumps in with adverse weight on the dog, the 2yr old can easily wheel the 4yr old around. The weight of these carriers is on the wheels so it should be the same force/weight as being attached to a pram.
Attach it to the dog's rear axle