Is petechiae a common reaction in some children when fighting off everyday illness or infection?

PETECHIAE is...: Bleeding into the skin, doesn't blench on pressure. If associated with fever without localizing signs, indicates high risk for serious bacterial infection , e.g. Bacteremia, sepsis, meningitis. Viral rash, on the other hand, very common; seen as macular or maculo-papular rash that blenches on pressure. I assume, you are talking about viral rash. If petechiae, must see a doctor.