I'm a big proponent of Mustela products. We used the no-soap products to gently wash the affected area. Not only did the baby smell terrific, but his skin cleared up beautifully. Just keep the area clean and dry and usually the skin clears up quickly.
My son who is now over two still gets flare ups, but this is due to his allergies. If you have an older child, make sure that the "acne" is not an allergic reaction to something he's ingesting.