My 12 yr old son has slapped cheek rash but just on his arms.he is itching madly all over , antihistamine doesn't seem to help .what can i do to help?

Cortisone cream: You can use cool compresses or otc cortisone cream. Benadryl (diphenhydramine) may work better than Claritin or Allegra for this type of itch. Which antihistamine you pick can matter. Children are typically less sedated than adults (by sedating antihistamines). If this is an allergic rash and not a virus, your pediatrician might prescribe oral steroids, if the itching doesn't settle down or the rash gets worse.

Topical steroidal cr: Perhaps a stronger anti-histamine (if using benadryl- then perhaps Claritin or zyrtec or allegra) but adding a topical steroidal cream such as Hydrocortisone (available as a 1% cream) applied to the areas will give some relief and even perhaps some Benadryl (diphenhydramine) cream.

Fifth's Disease: Your description is of "fifth's disease." this is an exanthem (rash) caused by parvovirus b19. The facial rash (slapped cheeks) & truncal rash (lacy) last about a week. Nothing other than cool compresses or oatmeal baths help the skin, since the rash is inside the skin from an infection. Systemic antihistamines may help, but topical creams play no role in helping this condition.