Important note - the dog is huge (Newfie-Bernese-Poodle) and the child is small (< 30 pounds). Also, the dog has a very mild temperament, is familiar with the child (same household,) and has proven to be extremely safe with this child, even to the point of ignoring fingers in mouth and near eyes.
My understanding is that the primary risk of a child riding a dog is that the dog freaks out and/or the rider falls off. That is not my main question.
What I am concerned about is the physical stress of having even a diminutive rider - dogs were not bred as mounts (citation needed), and I'm uncertain how well their body structure will react to a concentrated weight on the back. I figure that time and activity level could also have an effect: what may be fine to carry for 30 minutes could be dangerous to carry for 5 hours.
What harm could come to a dog from having a weight < 1/5 its own body weight placed on its back, and is there a way to mitigate this?
The only halfway-credible resource I've found says that most dogs can handle ~10% of their body weight without much of a problem, with the number varying significantly based on breed. (Source = https://www.cesarsway.com/dog-training/training-tools/dog-backpack-dos-and-donts) Making or otherwise obtaining a saddle is within reason here, so a backpack seems like a decent comparison.