Hi. Thank you for reaching out to Health Care Magic. There are several causes of rash over the abdomen or the buttocks. Based on the age of 9 years, here are a few possible causes- Rash Allergy Insect bites Medication allergy Skin infection. Most causes of a rash are seen because of release of histamine from the body. This can cause the skin to itch, redness to occur, and swelling. This is because of the leaking of the vessels seen with histamine. Here are a few things that many people find useful for this - Using cold compresses over the area Drinking lots of water Using calamine lotion Using a anti-histamine cream Keeping the skin dry In case the rash persists, getting a biopsy or a WBC count would help to get to the cause. The doctor would then suggest antibiotics or steroids to help with the symptoms. I hope this helps. Please reach out to me if you have any other questions. Dr Vignan