Thank you for the opportunity to hopefully help you and your dogs, and to dispel a common myth about dogs eating grass.
I often hear from clients and members of the public about dogs who they believe eat grass to induce vomiting. It is true that dogs eat grass (don't you know it!) as well as other debris and that they then often vomit as a result. It is not true, however, that dogs
make a conscious decision to induce vomiting and eat grass as a method to do so. That would imply a level of understanding (and medical skill!) beyond what a dog is capable of. I surmise that the typical weaning of a puppy at 8 weeks with adoption to human families shortly thereafter results in a shortening of the educational period available from the puppies mother. That period is when transfer of valuable information on what to eat and what not to is transmitted. It would be interesting to know if wild canines have as high an incidence of pica (eating non-food items) as our domestic dogs. While no one can say for certain why dogs eat things other than food (sometimes unspeakable things!) it is undesirable that they do so and it is up to us to prevent them from doing so.
Some things may taste and smell good to dogs but after ingestion may remain in the stomach a long time and may irritate the lining of the dog's simple stomach leading to vomiting. Grass is not digested by animals such as dogs and cats, as opposed to complex stomached animals such as cattle, goats and sheep. The former may like it, but shouldn't eat it.
In your case, your Papillon, having a smaller stomach, may be more irritated by the presence of grass in it than your larger Border Collie. Alternatively, the vomiting and the diarrhea may be only incidental to the grass eating. Chronic diarrhea is not typically a feature of eating grass, though contamination of that grass with parasites or bacteria may lead to disease and diarrhea.
An under considered and under-diagnosed set of gastrointestinal diseases falls under the category known as Irnflammatory Bowel Disease (IBS) a close cousin to the "IBS" (irritable bowel syndrome) of people. Depending on what level of the gastrointestinal tract is affected, the signs of IBD may be vomiting, diarrhea or both in various combinations.
Alternatively your dog may have a dietary intolerance or even an allergy to a particular ingredient or ingredients in a food or foods. Sometimes natural material toys may carry
bacteria. Often lake environments are contaminated by animal waste carrying parasites and bacteria which may cause these symptoms as well.
In sum, chronic vomiting and diarrhea in a dog, young or old should be worked up by a veterinarian through testing and a cause sought and addressed. The possibilities are simply too numerous and complex to guess at and I encourage you to start down that road as soon as is practical. Blood work, urinalysis and a feces examination for parasites and bacteria including giardiasis is warranted here.
Good luck and please do let us know how it turns out for your dogs! Definitely do begin discouraging ingestion of grass and use your leashes to enforce it!
Dr. Arnie Goldman