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?

Atopic Dermatitis. You are pointing to the cubital area were atopic dermatitis is common. It is difficult to DX skin lesions without being seen. Try putting hc 1 percent for allergy otherwise see md.
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.