My 3 year old daughter has what appears to be a cluster of pimples on her shoulder/neck. what could this be?

Skin irritation: Presumably she may be having long showers and hot water falling on her back may have removed a lot of fat from the skin -making it dry, itchy and rough and with pimples. If so try to lessen baths & esp try to avoid hot water baths. Use luke warm water instead &quick baths. A moisturizing cream may also help. If not better or if I surmised incorrectly see a dermatologist for proper diagnosis.

Hard to say: This could be a contact dermatitis like a poison Ivy eruption (appears ~ 18 hrs after exposure) or it could be shingles. Skin issues often need to be seen to be properly diagnosed.