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Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help. Please give me a moment to review your concerns.
With multiple young cats with chronic, intermittent, loose stools parasites and abnormal bacteria are probably the most likely cause. Because eggs and cysts are shed intermittently they may not show up on every sample. Your veterinarian needs to look for protozoal parasites (like giardia, coccidia, toxoplasmosis, or tritrichomonas) as well as the usual worms. So although it would be troublesome for you the quickest way to determine why they have loose stools and know how to treat them is likely to be submitting fresh stools samples for testing. I highly recommend that a fresh fecal sample be submitted to the lab rather than checking it in the clinic as these can be hard to find, and sometimes we need to check samples from several days in a row to find them. They can check for abnormal bacteria as well.
Here is a link to very thorough testing option for kittens with chronic diarrhea:
You might also try worming them with a broad spectrum wormer like Panacur (fenbendazole) which covers the most common worm parasites and Giardia. Unfortunately it doesn't cover other protozoa, but it is a good start.
Since they do go outside they should be wormed every 90-120 days as a preventative measure.
Best of luck with your kitties.