Be aware that if your ex (or anyone) is giving you money then you have to declare this to the benefit people and they will reduce your benefit accordingly. Wherever possible get people to give you things (clothes/toys/food) not the cash/money.
Also the CSA will most likely get involved if you are on benefits and he will have to pay maintenace but you will not get to see it apart from £10 a week which is changing to £20 a week at some point. I mention this because if your ex gives you money and then gets a call from the CSA he may well say 'I already give her money' and that will land you in it with the benefit people who will reduce your benefit and you'll have to pay back the value of what he's paid you.
On benefits you basically have a fixed income.
Work out what your expenses are. List them into essentials such as electric/gas, water, phone. Then the not entirely essentials like insurance, tv license, running a car, internet provider. Then all the luxuries such as sky tv, magasine subscriptions or anything like that, kids activities.
First of all the non essential stuff has to go. Now work out what you should pay per month on average on all your bills and then ask to pay monthly or go on a meter for electric etc. Then work out what you have left for food, clothing (charity shops and hand me downs are where you should be shopping i'm afraid). If the kids do any activities (scouts/brownies, swimming, music) ask the leader/teacher about help for people on low incomes or ask their dad to pay for these. Also school trip etc don't be afraid of talking to school as they often have things in place for people on low incomes and can help pay for activities etc.
Make sure you are claiming everything you possibly can, school dinners, free prescriptions, help with school uniform etc. The benefit people won't tell you what you can claim, you have to ask. Sometimes it's better to ask the CAB for help on that kind of stuff.
Once you have your essentials sorted out list out everything you buy each week/month food shopping wise. Give it a good hard look over and see where you can cut costs. If you have any money left then think very hard about what you can spend it on. You might want to buy xmas saving stamps and start putting aside small things each month. I have already got my son 2 xmas presents. Don't let it creep up without thinking ahead.
Find activities you can do with the children that don't cost anything. Nature walks, visits to the library, free museums etc. Being broke doesn't mean you have to sit home all day!
You're doing the right thing planning ahead though. Never put your head in the sand, and never be afraid to ask for help!
best wishes
Gilly