His father is a doctor who treats children and infants.

His father is a doctor who treats children and infants. Correct Answer Paediatrician

One word substitute is Paediatrician.

Dermatologist: a medical practitioner qualified to diagnose and treat skin disorders.
Gynaecologist: a physician or surgeon qualified to practise in gynaecology.
Podiatrist: a person who treats the feet and their ailments; a chiropodist.
Paediatrician: a medical practitioner specializing in children and their diseases.

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