Please call the pediatrician instead of taking our advice.
Just in the off chance it's not a black head (could be the start of a mole or a more serious medical condition), I'd call the pediatrician's office to be sure.
Our daughter had what looked like a pimple on her back last week. She's two, and it really seemed to hurt. Thank God a friend mentioned it looked like her son's MRSA - a serious staph infection. We called the pediatrician who worked her in that day, confirmed MRSA and put her immediately on an antibiotic.
Derm issues are really hard to diagnose, so let your pediatrician guide you - even if it is acne. I, too, have struggled with acne most of my life, but a 3.5 year old getting it is really uncommon.
Good luck!