When should a 1 year old's diaper rash be looked at by a doctor?

As necessary: If you are unable to clear your babys rash by standard means consult your pediatrician.

If no improvement: Moisture, heat and lack of air circulation may cause skin irritation which may get over infected with bacteria or fungus. Baby may need a medicated cream, so if no improvement with careful drying and vaseline/zinc oxide based ointments, see a doctor.