Do you have Canesten or Daktarin in the house? (Or any thrush or athlete's foot cream) That's what GP or pharmacy will give you. Apply it two or three times daily and it will start to improve in a few days. You don't need to cover it once you are treating it but wash hands and towels carefully.
Impetigo usually has a yellow crust or yellow watery discharge which you can't see in this pic but it might have washed off in the bath so have another look later once her skin is totally dry. You need antibiotics for impetigo.