The W-4 you fill out for your employer affects how much tax the employer will withhold from your paychecks. The W-4 does not go to the IRS or dictate how you file your tax return. If you show on your W-4 that you have a dependent child then less tax will be withheld from your take home pay.
When you prepare and file your tax return on the Form 1040, that is when you will actually claim your child as a dependent in order to receive the child-related credits. Enter your child's information--including a Social Security number-- in My Info to claim your child as a dependent.
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