I am so sorry OP. But perhaps you want good advice, not sympathy. A couple things come to mind:
a) of course, file for support
b) Washington is a very expensive city; does your job bind you here ? Do you have any family in another part of the country where you could stay short term ?
c) What do you do professionally? Any chance you could take a year or two to get a masters and up your income prospects ?
d) how old are your kids and how soon will they start school ?
e) what about your Ex's parents; are they upstanding people who will stand by the mother of their grand children and help w/child care or expenses ?