My 3 & 6 yr olds both got some red bumps on stomach and slowly spread to chest and arms and face. they do not become blisters. they also don't itch.

Should see a derm: Either a type of eczema, or possibly a type of wart called molluscum, which seems most likely.A dermatologist should be able to sort out.

Contagious v exposur: Since they both have this rash, you must consider something contagious like a virus , impetigo, or something they were both exposed to like insect bites, although most do itch.Best to take them to their pediatrician for a diagnosis and proper treatment.