What can I do to keep my child from scratching the rash?

Keep it covered. Itchy rashes are difficult to leave alone, especially for kids. Covering the rash, so the child cannot scratch it, and keeping the hands covered are techniques one can try. Identifying the cause of the rash to prevent it from worsening may also help.
Cover it. Having your baby wear long-sleeved onesies or other clothes that cover the rash will help. Often a moisturizer can help soothe the skin. Finally, discuss the cause of the rash with your pediatrician - treating the rash will make it less itchy. Benadryl (diphenhydramine) can help with itching so ask your pediatrician whether it is safe to give your baby.