Well... first off...you never start out with the idea that it's an incurable disease. You start out with the idea that it's a treatable illness, and then work work to a diagnosis of exclusion.
Secondly, Lupus has about a gazillion differentials (okay, maybe 200) of other things that it could be that are NOT Lupus (the vast majority easily treated). Even something simple like asthma has over 80 differentials to exclude... Lupus is a multisystem disease... so it has a LOT more. So don't fret. Even in the rare case that it might be Lupus (nearly never is), it's going to be awhile. Every treatable condition needs to be ruled out first, as well as any untreatable condition that might just be mimicking it.
So BREATHE. Because if what you're reading means it sounds like Lupus... about 200 OTHER things also sound like lupus... which mean they sound like what your granddaughter has.
Really... this a hot red rash on the knees could be anything from rug burn, to contact dermatitis, to juvenile arthritis, to a bacterial infection, to, to, to, to (insert 200+ possibilities).
What docs usually do, is to clear the most probable things first (an antihistamine in case it's just an allergic reaction -which can cause redness and swelling- ... and an antibiotic to stomp on a bacterial infection like staph). Doing these two things will rule out over HALF of what it "could" be. They probably did an antihistamine in urgent care? To check for contact dermatitis (allergic reaction through the skin)? No effect was shown, so they sent you home with an antibiotic and are having you check with your Ped.
What they USUALLY say is "If it's not better by such and such a time come back in." which means that it's not a bacterial and then need to start doing more testing. MOST of the time, people don't need to come back in, so if you don't... they assume it was a bacterial infection cured by the antibiotic.
AKA if the antibiotic is having no effect, DO call and let the doctor know that.