Why do pimples appear on my butt?

Moisture is often a cause of pimples, clogged pores. If your toddler has pimples on the buttalks, I would suggest using a anti-bacterial soap on the buttalks only. Make sure to change diapers frequently to prevent extra moisture. Sometimes pimples can be caused by a rash, it can be caused by the brand of diapers, or laundry soap even. It's best to leave the pimples alone, do not scrub or bother them, and wash the buttalks often, with the anti-bacterial soap to clean and dry them out.