Hi Gary,
this is not rocket science, and it doesn't matter how old your daughter is. You haven't mentioned how or where this itch occurs. It can be from not rinsing her completely of soap or shampoo in the bath, or from urine or fecal matter not properly cleaned from the crotch area during the day. Does she wipe, or attempt to wipe herself?
A good diaper rash ointment or powder properly applied, and paying strict attention to your daughters hygene needs like changing her diaper frequently, can minimize problems like you describe