Think long term here...
Yeah taking the sick kids that day helps her out... but what about in two days when you are sick and cannot take them? When they lose their sitter for days, even weeks, because of what you caught from them? That's not helping in the long run.
And if you DO take the kids, when you know you've been exposed, then you run the risk of giving it back to them... and then they give it back to you and on and on it goes... not even counting if Mom gets it and as a result misses work and causes them financial troubles.
Saying no one day can save days and days, even weeks, of illness, missed work, etc for all later.... including YOUR family who may be contaminated by you later on... and how many can they then spread it too... and so on. Isolation is the only way to stop a bug from spreading... that's why so many places (schools, day cares, etc) have rules like that in place.
I know that sounds dramatic... but it's a fact that they caught it from somewhere because someone didn't isolate... and because Mom knowing hers were sick didn't isolate now you are ill...
You aren't responsible for their choices, not at all, but you are responsible for your own... you mentioned that you will be forced to skip your plans to protect your family... and that sucks, but is the best choice to protect those you care about... well... do you care about these kiddos? You seem to, VERY much... so why would you risk reinfecting them?
Just something to think on.