With a rash, often a picture is worth a thousand words and sometimes it is hard to determine a rash over the telephone, so you may have to take your child into the doctor's office so they can take a look, but in general with a rash, my rule of thumb is that if the rash isn't bothering your child, it doesn't bother me. Rash can be caused by many things from simply dry skin to a more serious illness, so if the rash also has an associated fever, if it really hurts your child, if they are really itching, if there is any involved facial swelling or black and blue marks and your child really looks sick, then they should be seen right away. Rashes that are due to an allergy, for instance, something you ate or something you came into contact with, often appear right away and may even be more severe. They may cause itching and make your child really uncomfortable. If there are any signs of trouble breathing or facial swelling, then it is important that your child be seen right away. Otherwise you can try giving your child a dose of an over-the-counter anti-histamine and the rash may go away on its own. A rash that persists for hours and hours or days and days is usually less likely to be due to something your child ate or drank, or something they touched, but if you have any concerns, always see your pediatrician. They can take a history, look at the rash and talk to you about ways to make the skin better.