Yes, they will automatically add the tip. An infant counts as a person on an ADR. All persons at the table should be registered, whether they will be eating or not. All parties of six or more have the automatic 18% tip added.
All tipping questions are referred to the Tipping Information sticky thread at the top of the Restaurants board. For that reason, we close all active threads on the main board regarding tipping questions. The first post of the Tipping Information Thread has an FAQ that discusses the basic questions usually asked about WDW tipping policies.