DO NOT GIVE UP YOUR JOB (sorry for the capitals, but really, you are in a much better position as working lone parent than as a lone parent in benefits, for now and for the future, as you will need to go back to work eventually (when your youngest is 7) and the bigger the gap in your CV the more difficult to get a job.
If you are working for more than 16 hrs you are entitled to working tax credits and if in a low salary, you may even get up to 80% of your nursery costs covered by it. If you are working more than 30 hrs, you get an extra bonus of about £700 a year.
If still it's difficult to cope with expenses, you can apply for a reduction in council tax, and even for help to pay your rent (the amount you get or if you get it depends on your income).
As a non working lone parent you will only get £64.20 of income support a week, £53 aprox per child on child tax credits, and the £20 of child benefit that everyone gets. (you will be getting these anyway if you are working) Maintenance payments are deducted from the £63.20 so, for example, if your ex pays £40 a week for the children, you will only get £24.20 of income support, if he pays more than £64 per week, you get nothing. And trust me on this, living on £64.20 a week it's very very difficult even if they are paying your rent.
I found the Job Centre very useful. I know that you already have a job, but they were GREAT to point me in the direction of people who could help me sort child maintenance, improve my job prospects, etc. Initially I contacted them because I wanted to apply for a reduction of council tax, but definitively got more than I expected, not in the financial side of the things but in terms of surviving as a single parent, even in terms of finding support groups to help me handle life as a single parent.
Good luck!