What type of 'interdigital cysts' do your dogs have? There a couple of types. Most common in young dogs are either histiocytoma's or papilloma's (like warts) Histio's are caused by an increase in the dogs immune system, while papilloma's are caused by a virus. My guess, if you are seeing it in more than one dog, is that it's a papilloma. They go away on their own, but are highly contagious, especially to young dogs.
However, interdigital cysts can also be cancerous (histiosarcoma). Epsom salt won't fix that.