Revise your health policies in the contract and have them all resign them.
My parents have to pick up within an hour when I call.
At this time of year I always put out a newsletter talking about health policy reminders and the importance of not sending kids in sick, and review my exclusion policies. I then follow it up with a statement about how if I or my own kids get sick from their negligence, then I will have to close, which inconveniences all of the families. It's a bit of an overkill and very blunt, but sometimes that's what they need to hear!
Send the kids home when symptoms appear, don't let them hang out for the day. It's not just about spreading the germs, but sometimes it's about the sick child needing one on one care while they remain in your care, and you can't watch them all effectively in that instance.
I have many teacher clients, and they can still be here in an hour, or send a spouse or alternate pickup person.
I make sure all undiagnosed rashes are followed up with a doctor, and they can't return until doctors note says they are not contagious. In my area in Ontario lately, there are cases of roseola/fifths disease/scarlet fever all over the place. It may be something or nothing, but have someone check it.
I excluse for vomitting/diarrhea for a full 24 hours after the symptoms have stopped. If the kid comes back in with diarrhea after exclusion, they go home again.
You have to set the tone for the behaviour. Stand firm, or they can find care elsewhere.