In my opinion kids riding on the dog's back is not exactly play you might want to encourage. You want let your dog know that the kids are superior to him not play mates.
I disagree. Probably not great for the dog as the kid gets heavier but dogs need to know Kids are unpredictable, and a dog needs to learn to be tolerant of them. If my son decides to hop on our dogs back, he let's him and that's that. If my son decides to give our dog a cuddle, our dog let's him. God, if DS wants to put his fingers up our dogs nose, pull on his ears and prod at his eyes he let's him! But of course there are times you draw a line purely because it's not nice of the dog. Eg: having a toy lawn mower plowed into you. Our dog still let's ds do that, but I don't because it's just mean! (same with the eye poking/ear pulling). DS also "gives" mannix stuff. I've watched him line up an assortment of toys, his own drink, hand things to mannix and mannix gently takes them from him or just watches (in amusement or tires annoyance, I'm never sure!)

When we first got mannix we were kidless however my sil had children so we always "roughed him up" from the get go so he'd be used to these "toddler" things. Pulling on his ears randomly, at his tail (obviously not to cause pain) so he would be tolerant of it if they did it.