The child that you had together will qualify you for Head of Household. Neither your GF nor her child would have qualified you for head of Household, because they are not related to you. Your GF and her daughter may be your dependents because they meet the six tests for being a dependent:
1. Closely Related OR live with you ALL year
2. His/her gross taxable income for the year must be less than $3,700 (2011)
3. You must have provided more than 1/2 his support
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 file a tax return claiming someone else as a dependent
6. He must not be the qualifying child of another taxpayer
Because your GF's child is not related to you, she cannot be your "Qualifying child" for such things as the Earned Income credit(EIC) or Child Tax credit(CTC); even though she can be your dependent and you get her $3700 exemption (deduction).