My daughter has a very itchy bum and she scratches it quite a lot. What could be the problem?

Does she still wear diapers? Some diapers have these elastic lining near the thighs for proper grip and to avoid leakage. It could be the elastic or the quality of diaper. Leave her open without diaper for sometime in the day in your presence. Sometimes this can happen if urine is very acidic. So pls see ur pediatrician

Is it possible that her bum area doesn't get properly dry and remains wet after a shower or clean up? Make sure you help her dry properly each time she uses the washroom. Other than that, apply a good cream and if nothing helps visit a skin specialist