Since you have no income to report, filing Injured Spouse will make no difference for you at this point. The Injured Spouse divides your portion of income/withholding/dependents from your husband's. Since there's nothing to divide, you can file as usual.
In general, if you are reporting your own income and are filing as Married Filing Jointly, you can file Injured Spouse and it is your choice how many dependents you'll claim and how many will be allocated to your husband. TurboTax will ask you for that allocation when going through the Injured Spouse interview. Once completed, only his portion of refund will be applied to his unpaid child support based on his income, dependents and withholdings.
If you file Form 8379 with your original return, it may take 11 weeks to process an electronically-filed return or 14 weeks if you filed a paper return. If you file the Form 8379 by itself after a joint return has been processed, then processing will take about 8 weeks. To avoid delays, be sure to follow the Form’s instructions.
I am enclosing a link to the form 8379 and its instructions.
http://www.irs.gov/pub/irs-pdf/i8379.pdf