It's complicated.
You
may file using Head of Household (HoH) filing status. The child that you share, with your GF (and I assume lives with you) is your qualifying
person for HoH.
The
non-custodial parent may claim his children for their exemption ($4050
deduction) and the Child tax credit, if the custodial parent releases the
exemption to him, But he may not count those children for the Earned Income
Credit (EIC), HoH filing status or the day care credit, because the dependent
children do not live with him.
You
may claim your girlfriend and her child only if they lived with you all year
(365 days/nights). The
child (and your GF) may qualify as a dependent, but because he is not related,
he cannot be a qualifying child for the earned income credit, child tax credit
or HoH filing status.
There are two types of dependents,
"Qualifying Children"(QC) and standard ("Qualifying
Relative" in IRS parlance even though they don't have to actually be
related). There is no income limit for a QC but there is an age limit, a relationship test and a residence test. Only a QC qualifies a taxpayer for
the Earned Income Credit and the Child Tax Credit. There is a special rule in the case of divorced &
separated (including never married) parents that allows the non-custodial parent to claim the Child Tax credit, in addition to the exemption.
A person can still be a standard dependent ("Qualifying
Relative" in IRS parlance even though they don't have to actually be
related), if not a
Qualifying Child, if he meets the 6 tests for claiming a dependent:
1. Closely Related OR live with the taxpayer ALL year
2. His/her gross
taxable income for the year must be less than $4,050 (2016-17)
3. The taxpayer must have provided more than 1/2 his support
In either case:
4. He must be a US
citizen or resident of the US,
Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a
dependent of his own
6. He must not be the qualifying child of another taxpayer
He must have a US
social security number or tax identification number (TIN)
In
addition to the above requirements, to claim your boy/girlfriend's children,
they must meet all of the above requirements and:
--- your boy/girlfriend must not be required
to file a return,
--- he/she does not file a return claiming the
children
