7 year old developed itchy rash first on back of neck later in day it spread to arms, stomach and legs no other symptoms, what is it and what to do?

Hard to say: If the rash is moving from a local to a general distribution it is unlikely a simple contact dermatitis like poison ivy. Generalized rashes can be secondary to an infection (virus or bacteria) a food reaction or bug bites. Without examining the rash it is hard to speculate on a cause or specific treatment. If it itches, Benadryl can be given for comfort.