What can I do?

Almost 2 years ago I discovered a female stray at my job. Our family had recently moved to the country and we have pole barn I believed to be safer for her than the busy city streets near my job. We should have gotten her fixed but she was pregnant at the time. She has since had 2 litters. A total of 3 males from the first litter and 3 males and 3 females from the second litter. We have also had 1 male (a kitten when he appeared)that are very friendly. Another very feral male, 1 female and 1 male that I would call moderately feral have adopted us since. They are all fed the basic outdoor cat food and up until recently they have all done well with it. We ran out of food one late night and had to unexpectedly buy a different brand.They have since switched back to their previous brand. Coupled with that and a TOTAL screw up on my part where I thought I was being nice and gave them warm milk for a couple weeks every few days while it was cold, some of them have developed diarrhea. A couple of them are even pooping on my porch as they are eating. There is a little mucus noted in some of it. They otherwise have no symptoms. They are still playful, no eating grass, and seem completely normal.
What can I do for them short of a vet visit? I have many children and not a lot of money. The reason I've kept them all in the first place is not only are we cat lovers but the shelters and rescues in the area are at capacity and will accept the pets but they are not treated if they seem sick then they send them to a kill shelter. You know what happens from there. Thanks in advance for any advice you may have

OK so I am guessing that the milk treats are over. They are on their old regular food - which is?  Only other thing I can think of is worms, which come from fleas, which most likely would be in the barn, or from rodents that they mot likely catch as outdoor kitties.  I think taking a poo sample to vet is rather inexpensive if you are unsure. Although, your vet could probably prescribe a wormer for cats without a visit, as worms are common, especially with multiple cats and kittens. Oh and what kind of food are you feeding? Sometimes dry food goes rancid very quickly due to the flavor enhancer that is sprayed on at the end of process. That enhancer is some nasty stuff!

you need to find a low-cost spay/neuter in your area.