Usually, the custodial parent gets to claim the qualifying child (or qualifying children) as dependents. This is true unless you have a divorce decree stating otherwise.
If you feel that you should be able to claim the dependent, you’ll need to print and mail your return. You won’t be able to e-file. The IRS won’t allow two different people to e-file using the same dependent Social Security number (SSN).
The IRS will then send a letter to both of you to determine who gets to claim the exemption for the qualifying relative.
If you can’t agree on who claims the child, the tie-breaker rules apply.
Under the tie-breaker rules, the child is a qualifying child only for:
The parent with whom the child lived the longest during the tax year
The parent with the highest adjusted gross income (AGI). This applies if the child lived with each parent for the same amount of time during the tax year.
The person with the highest AGI if no parent can claim the child as a qualifying child
A person with an AGI higher than any parent who can claim the child as a qualifying child but doesn’t