If you are legally married you have the option to file as married filing jointly or married filing separately. You are not required to live together to file a joint tax return. As SuperUser Rainman said, the six month thing you cited is for a situation in which a custodial parent who has income could file as Head of Household and claim the children as dependents if the couple lived apart for at least the last six months of the tax year. According to you, you have no income--so filing a tax return as Head of Household would be of no benefit whatsoever to you. Without income earned from working, you would not qualify for any child-related credits or any sort of tax refund.
Your estranged spouse should have your permission to file a joint return. He can file electronically without you physically signing anything. You would be prudent to give him permission to do so, so that he can get the tax benefits of filing a joint return. That means he gets child-related credits and perhaps is in a better financial position to pay the child support you and the kids live on. It does not sound as if you have anything to gain by objecting to a joint return.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
