What are the potential risks (to the dog) from having a child ride it like a pony?

Question

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.

Answer 1

I was not able to find anything 'scholarly'. However, I ran into enough enthusiast sites and reddits to be confident that the general guidelines are followed without problem.

Summarizing the main points of the sites I found

1. 10% is safe for almost any breed. 25% is safe for most breeds. 25+% can be reached by some breeds with proper conditioning.
2. Age and overall health condition can lower this number.
3. Proper fitting and balance (loose enough to put in a finger or two) is very important
4. Never put a burden on a dog that's less than 2 years old (not yet fully-developed.) Doing so risks serious joint damage.
5. Be wary of over-heating. A pack or saddle will hold in some heat and increase effort, putting the dog at greater risk of heat exhaustion.
6. Do not sharply increase the load - increase weight gradually.
7. If carrying weight in "rough" territory, consider getting a pack with a handle on top so that you can easily pick up your dog.

Sources:

https://www.peacebonepet.com/blogs/news/rei-hiking-or-backpacking-with-your-dog

You should also start with lighter loads. It’s safe to work to up to one-third of your dog’s weight if your dog is in healthy physical condition. For dogs who are older or in poor physical condition, consider leaving them at home with friends, They’ll be much happier… and safer, too.

https://www.rei.com/learn/expert-advice/hiking-dogs.html

A maximum of 25 percent of body weight is a rough guideline, but factors like age, size and strength will alter that up or down.

http://dogsaholic.com/training/how-to-put-weight-on-a-dog.html (talking about using weights as a workout tool to bulk up a dog, so the max % is in terms of starting.)

Generally, a weight vest is not meant to stress the dog and to get him into submission which is why it should be no more than 10 – 20% of the dog’s bodyweight. Dogs under two years are not yet fully grown and should not be burdened with weights. Overworking dogs that are still at puppy stage can damage their joints.

Talking specifically about riding like a pony

Riding bareback could pose issues with weight balance, which is a big no-no for dog burdens. A well-balanced and well-fitted saddle shouldn't be any different than a regular pack. Standard weight limits would then apply.