I would suggest letting the parents know what you consider mild illness (sneezing, slight runny nose, mild cough...) and then telling them that it is unfair to the other children to have to care for an ill child. It takes up a lot of your time and also puts the health and welfare of the other children in your care into jeopardy. I would just let them know that you didn't consider those things when you initially told them it was okay to send their sick children to you. If a parent does choose to medicate a child and send them to you anyway I would call them at work and ask them to pick their child up as soon as the meds wear off and the fever or symptoms comes back. If a child is too ill to participate in your regular daily activities they should be at home with their parents which is where most children would rather be when they are sick anyway.
My take on parents and sick kids is - Didn't momma tell you there would be days like this before you had kids and decided to continue working?
Don't let them guilt you into caring for their sick kids. I have learned in my 9 years of childcare experience that when it comes to working parents...give them an inch and they will take a mile. If you tell them you'll take a mildly sick child they will send you an extremely ill child and tell you that they didn't seem that bad this morning...ya right!
Be firm and let them know that you have changed your policy. Be very specific as to what you will and will not tolerate.
Good luck. I know how hard it is to try and make changes but sometimes you try things and it just doesn't work out like you anticipated.
PS - Even if they do follow your new guidelines remember that childcare is messy business and most childcare providers will end up being ill a good part of the time. I don't take any sick children at all here and I make sure everyone washes their hands as soon as they arrive from school but so far we've still had the stomach bug, 2 colds, a throat infection and I am currently battling pink eye (Gross!). Most of the time kids are contagious even before they begin to show symptoms so often they've managed to contaminate everyone before mom and dad even realize they are sick.