My 20 month old child has a dark red rash with bumps on the inside of on arm at the elbow, and nowhere else. what can be applied to it or be done?

Difficult to answer: Attempting to diagnose & then properly treat rashes "over the phone" or without actually inspecting is usually ill-advised. Nonetheless, based on my experience of nearly 30 years of office-based pediatrics, rashes that most frequent the elbow area include: atopic dermatitis, eczema, tinea(ringworm), abrasions, contusions, & contact dermatitis(poison ivy). So, therapy remains unclear w/o more info.