There is an exception to every rule, in this case, the rule that man-walks-dog and not dog-walks-man. Where the dog is bigger and more experienced than his toddler companion, roles may become reversed and we may have the dog walking the man-child.
That, clearly, is what happens in the case of these two "Best Friends." But given that the little boy holds the leash we may, at first, mistake the order of the association as conforming with the conventional man-walks-dog scenario. But wait until the little boy encounters a commonplace formation of nature, a puddle on a lonely road, and the true nature of the partnership comes to the fore.
Best friends go for a walk
The boy drops the leash for a brief course in nature studies while the dog waits patiently for him to satisfy his curiosity, much like a parent conscious of the need to foster learning experience opportunities for the young, or like the man walking his dog, who waits patiently while the dog sniffs around the bush, and raises a hind leg to make a puddle.