Causes of external ear infection or otitis externa
If your child has an external ear infection, the lining of her ear canal on the outer side of her eardrum will be infected.
Children who swim a lot often get external ear infections because water can stay in their ears after swimming and cause irritation.
External ear infections might also happen if your child’s ear canal is damaged by using cotton buds or scratching. A secondary bacterial or fungal infection often develops when this happens.
Children who have skin problems such as eczema or dermatitis are more likely to get external ear infections.
The medical term for external ear infection is otitis externa. You might also hear it called ‘swimmer’s ear’.
Source: http://raisingchildren.net.au/articles/ear_infection_external.html