Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 25:

Qualifying Widow(er) With Dependent Child
If your spouse died in 2015, you can use married filing jointly as your filing status for 2015 if you otherwise qualify to use that status. The year of death is the last year for which you can file jointly with your deceased spouse.

You may be eligible to use qualifying widow(er) with dependent child as your filing status for 2 years following the year your spouse died. For example, if your spouse died in 2014, and you have not remarried, you may be able to use this filing status for 2015 and 2016.

This filing status entitles you to use joint return tax rates and the highest standard deduction amount (if you do not itemize deductions). It doesn't entitle you to file a joint return.

Eligibility rules. You are eligible to file your 2015 return as a qualifying widow(er) with dependent child if you meet all of the following tests.

• You were entitled to file a joint return with your spouse for the year your spouse died. It doesn't matter whether you actually filed a joint return.
• Your spouse died in 2013 or 2014 and you didn't remarry before the end of 2015.
• You have a child or stepchild for whom you can claim an exemption. This doesn't include a foster child.
• This child lived in your home all year, except for temporary absences. There are also exceptions for a child who was born or died during the year, and for a kidnapped child.
• You paid more than half the cost of keeping up a home for the year.