Toddler Rash

I babysit an 18 month old boy and he has a spottie red rash all over his body.  Absolutely no fever.  The spots on the outside of his forearm are worse than the inside.  No spots on his underarms, some spots on his hands.  I'm thinking it's just a heat rash of some sort.  He's not hot an clammy.  Doesn't seem uncomfortable, no loss of appetite.  He's pretty much normal with the exception of the spots.  I'm not overly concerned but would like to know what this is or if it is contageous -- I have 4 other boys in the house too and don't want to expose them too long without a diagnosis.