Ruby is 2 years old and is seen today with her father as she has had a one week history of runny nose and has complained her throat hurts. This morning he has noted a rash on both cheeks but nowhere else on the body. Ruby is well in herself and eating, drinking, passing urine and opening her bowels as normal. She has no other medical history and is up to date with immunisations. She is not lethargic and on examination today is playing happily in the consulting room, has normal observations and an unremarkable respiratory and throat examination. There is a diffuse erythematous rash on both cheeks.
What is her most likely diagnosis?