The ER Doc confirmed an ear infection right? So, penicillin for that.
He also told you your son had a viral rash. For a virus there is no treatment except bedrest and fluids.
Measles is........ a viral rash.
http://children.webmd.com/tc/measles-rubeola-treatment-ov...
And the treatment for measles is..... bedrest and fluids.
So your son WASN'T actually treated for the wrong thing.... the Dr just didn't diagnose the viral rash as anything more than a viral rash. It may have needed time to progress in order to be correctly diagnosed. I used to be able to diagnose an ear infection in my daughter SEVERAL DAYS before it could be confirmed by the Dr - so I learned to manage the symptoms at home and wait until I knew it was far enough along that it could be confirmed and THEN go to the Dr and get the antibiotic.
If your son as a undetermined viral rash - that could be anything... and you never know if it's contagious or not, until the rash goes away. That's why you are "supposed" to keep your kid out of daycare until 24 hours after medication. In the case of virus you can't do that (no medication) so you have to keep them home until they are not symptomatic (ie the rash goes away).
I'm surprised daycare took him with a undetermined viral rash (even saying it's 'from the ear infection' still means they don't know the cause, so it's undetermined). Now, as working moms we all have to make decisions so you did the best you could do.
But it's certainly not the Dr fault other kids were exposed to measles.
but of course you're upset - we all get cranky when we can fix our kids immediately :-) I hope he feels better soon!