Unfortunately, your timing is considerably late for most grant programs. The Mississippi Development Authority ( http://www.mississippi.org
) had a few Homeowners grants - ( see http://www.mshomehelp.gov/
) but the deadlines on these past March 15th of 2008 but they would have only been able to go to repair of Katrina-related damages and not an expansion of existing structure because of growing family (congrats on the growing family by the way :-))
Depending on your situation, their may be civic or church organizations that may help, and potentially federal funds that may be found. I do not have any specific information on these but if you want to post more information here or email me directly through my website, I may be able to help point you in the right direction.