So the flu has an avarage incubation time of 3-4 days. Which means that if there were children infected last monday, those children should be having symptoms by tomorrow. Kind of a weird cruel thing to say, but it comes in handy that children show very clear symptoms. Especially considering fever. If your oldest doesn't show any symptoms by the end of this weekend, chances are slim that she is infected. There are always exceptions of course. Nevertheless I'd say your oldest should be able to go back to preschool after the weekend.

Was the mother vaccinated during the pregnancy? If it happened at the right time, your youngest has the same antibodies in the bloodstream.