I was going to say "fifths disease" which is a pretty common rash outbreak in children.
Here's a description of it:
http://www.emedicinehealth.com/skin_rashes_in_children/pa...
My neighbor's daughter got it after a fever etc. Her skin looked laced with raised, faded red splotches that connected like a network of islands, all over her body. It wasn't scarey and it didn't bother her, but I think she took meds.
After reading that website I linked for you, benedryl does sound like a universal OK thing to do since these rashes seem to come after viral infections which last and pass in 3 days time, so might as well keep the swelling down and keep the child comfortable and hang tight if nothing else worse pops up.
Best of luck, TLC!
K.