Oh man!! That's so frusterating Alphaghetti, I am sorry to hear that your kids are now sick.
In my contract I use the Province of Alberta's definition of a fever which I found on the web site for Alberta Health Services (maybe the province you are in has a similar definition). Anyways AHS defines a fever as over 98.6 F. taken under the arm, over 99.5 F. taken orally, and over 100.4 F. taken rectally. I used this wording in my contract. I also don't allow kids to come to care after the have had shots except my policy is 24 hours. I think if this mother has the nerve to give you attitude then perhaps its time to think of termination...she is being just plain rude. I hope your kids feel better soon.