pediatrician

pe·di·a·tri·cian [p?dee ? trнsh’n] (plural pe·di·a·tri·cians or pe·di·atrists) noun
children’s doctor: a doctor who specializes in the care and development of children and in the prevention and treatment of children’s diseases. See pediatrician

Encarta® World English Dictionary © & (P) 1999 Microsoft Corporation. All rights reserved. Developed for Microsoft by Bloomsbury Publishing Plc.

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Note added at 5 mins (2007-05-11 21:18:36 GMT)
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or "specialist in children’s diseases" if you wish :)

Dagfinn Aarskog (www.whonamedit.com)Aarskog became a licensed specialist in children’s diseases in 1964, and of medical genetics in 1974. He became full professor and chairman in 1971, ...

www.whonamedit.com/doctor.cfm/351.html