I am not sure how does your baby's rash look like but if your baby is constantly scratching his face and has red inflamed patchy skin, it might be eczema. My daughter had bad eczema which start out as a small patch and soon spread to the entire face. You might want stop using any soap on her face in the mean time and start using water to clean her face frequently.
Need to see the picture of your rash mummy, but whatever condition best to stop any soap for bath and creams/oils. It also doesn't hurt to apply a thin layer of breastmilk on her face.
It could be due to heat/milk rash. Try to wipe the cheeks with cooled boiling water as often as possible. I recommend Four Cow Farm's Calendula Remedy to apply after the wiping.
thanks