Can small children ride dogs if the dogs don't mind?

I had a St. Bernard who lived a good long 10 years before ping away. Our breeder suggested we do not allow small children to "ride" him, not only to protect his back, but also to protect his hips. Large dogs are notorious for developing hip dysplasia, a painful and degenerative condition. One of our German Shepherd mix dogs got this and it was terrible to watch her get to the point where cortisone shots no longer eased the pain and she couldn't even get up to relieve herself. Dogs might not mind small children on their backs, I know my St. Bernard loved the little ones. They could crawl all over him while he was laying down and not fret at all. But, I never let anyone put their child on his back. I think it's great that your dogs get along well with your young siblings, but sitting on the dogs' backs is not good for them even though they don't mind it.