I defaulted on some credit cards after me and my husband separated and now have a collection agency threating to place a judgement on me and then take money out of my bank account which only ever has enough in it to pay bill and support my three kid. Can they do this? I have no way to pay them the money at this time and after they refused my $50.00 a month payments I just don't have any way to pay them without having my kids go with out food.

A collection agencey can not freeze your account only a court order by a judgement,but before that happens you will be served with a court date
How old are these debts? You will have to check the laws in your state
Some states allow wage garnishments and some don't,as far as freezing your account they proably can after they get a judgement

Texas does not allow garnishment of wages, but it does allow attaching bank accounts.

As DownUnder said, they can't do this without taking you to court. Check out the Bakruptcy & Debt, Credit and Small Claims forums for tons of similar threads from people being hounded by creditors. You will find advice to deal with this situation.