What can i put on a rash caused by sand and salt water on my four year old?

Rash: Neither sand nor salt water by themselves tend to cause rashes. Remember we all have salt water in our bloodstream. And sand is an inert byproduct of silicon, which is nonreactive. If there is a rash, it is probably from some other factor. With sand, if it is pushed across the skin, could cause an abrasion just like sandpaper. Salt water might be mixed with something that he is allergic to.

Hydrocortisone 1% cr: Hydrocortisone 1% cream, used twice a day for 4-5 days, is helpful for minor rashes that itch and irritate. This includes rashes from bug bites, from playing on rough grass, from rough carpeting, etc... If a rash doesn't improve quickly, seems to be spreading, or looks infected (with pus, oozing, skin pain, etc...), a primary care doctor needs to examine the rash.