Is it normal for a 2 year old to sweat only her head while she is asleep?

Yes & no: The head of an infant is relatively large to the other body parts, gradually reducing in relative size until adulthood. At 2 yrs of age the head is one of the largest surfaces where a child exchanges heat with the environment, It is also generally uncovered allowing this exchange to occur. It can be a normal vent or an early symptom of illness. If the kid is otherwise thriving I ignore it.