What Could Red, Raised Rashes On Cheek In Infants Suggest?

Two year old and now my 8 month old have What started as raised, warm and red areas (about the size of a quarter) on their cheeks. After 24-48 hours the areas turned blue/ purple. Doesn't appear anywhere else on their bodies and don't appear to be itchy or sore but is apparently contagious since the 8month old just got it a few weeks after his brother.