It is better to file Married Filing Joint when you have children because some of the credits may not be available to Married Filing Separate filing status. For example: Child and Dependent Care Credit, Earned Income Tax Credit, Adoption Credit, Education Credits,
As for filling out W-4, most likely you will not need to do "Extra withholding" on Line 4(c).
Tax withholding calculators help you get a big picture view of your refund situation by asking detailed questions.
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