Yup exactly!!
I'd have no problem telling the parents (in your case of a constant 'sickness') that come Monday morning you'd need to see a doctors note before caring for the child. As a mom of a little on in the house too I see and KNOW where you're coming from. Sorry, I know all kids get sick and it's bound to happen in group care but in MY house with MY child I make up the rules as to how much I will allow.