I think its a good idea as long as,as you said you're not forcing them or tying them to it and not doing it for too long as it is reasonaly high impact for the joints I would assume, I am sure they'll be fine... If humans can get on it and not break their legs I am sure dogs, who are way more agile than us, will manage just fine as long as the speed of the machine is adapted to them!
I suppose you will have to get it get used to it yourself and see if your dogs can get tempted

xx