Mine states that I exclude if "symptoms" are present. It's not my job to diagnose and say yes, this is the stomach virus that's been going around. But if it looks like a duck and it walks like a duck, it must be a friggin duck! I list the common contagious conditions and then also say that exclusion may occur if any other possible contagious condition is present (in case I missed something on my list) or if the child requires more time/attention than I can provide while still caring for the other children.
So tell her,
Dcm, Johnny has had D 3 times and needs immediate pick up. This is a sign of a contagious condition and is also a health hazard as his diapers are not able to contain the mess. I'll see you in about half an hour! Buh bye!
And if she argues again, questions your policies, or doesn't pick up in a timely manner, hand her her two week notice when she arrives.

Reply