Wow.
Um... without knowing more details, I'd be a little worried about going overboard here. The bumps (you didn't describe them, but apparently the child's doctor isn't worried) SOUND like they could easily be molluscum contagiosum. It is harmless but annoying, unless they are scratched and get infected or spread to areas of the body that interfere (like eye lids, etc).
As for the staph, it is everywhere. An infection just means that someone scratched or had an open wound/scratch that got the bacteria in it.
Here is what sounds like a logical progression to me: she contracted molluscom from a kid at school (common childhood virus with bumps similar to warts), they itched and she scratched, infecting the scratched/opened skin with staph bacteria--that can be found just about anywhere. She got the infection treated and keeps any infected bumps covered (standard care for molluscum). She scratched another one and again, staph bacteria entered the "wound" site and it became infected.
What you tell the parents of the other kids is the facts. Make sure you have them all before you go scaring the daylights out of them.
It doesn't sound to me like it is that big of a deal, so long as any infected bumps are covered and treated, and you are washing hands appropriately, etc. But then, I am a mom who has older kids and has "been around the block" a bit with some of these more common childhood things and probably less prone to overreact than a new mom with a baby.
If you want to refuse to care for the 2 part-timers while they have active infections, I see nothing wrong with that. That is entirely up to you. You may be uncomfortable with the additional worry/stress of being particular about keeping the sores covered, etc. And that is your prerogative. If you decide you are ok with it, then get the facts and share them with the other parents.
Just please get the facts before jumping to conclusions about how infectious and dangerous having these 2 kids around actually is.