What could this be on my 3yr olds bum?

Shes 3.5yrs old. She is potty trained but at night she wears a night time pullup just incase (shes a sound sleeper) Shes got these bumps that look like pimples and they wont go away. Her 2 bum checks are like covered in them and they get itchy at times. Any idea what it maybe or what i could use in them. During the day she wears underwear. Thanks