I have taken DD(3) to the drs about this. We saw a nurse who said it isn't measles or scarlet fever (no red/sore throat), but couldn't say what it was - possibly slapped cheek or a non specific viral rash.
As well as the rash shown in the pic she has very red blotchy cheeks and cold symptoms. No fever and she's well enough in herself, just like she has a cold really. However her cheeks and rash look so bad - does this look/sound like slapped cheek to those that have experience of it? DD is my eldest so I've not encountered all these illnesses before!