Help !!!!

jimd-f

firstly you need to clarify which scheme you are on; either CSA or CMS as this effects the calculation.
secondly, how long has your case been running and how old are the arrears.
because you are employed you are an easy target for them because they can take money from your wages so in their eyes they can do what they want with you.
to get any sort of reduction you need to speak to the right people. if your case is with the debt or enforcement department you have more chance of getting somewhere.
call them when you have your figures in front of you and ask them to reconsider your payments. they can take 40% of your net income but only normally do this to people that upset them by their manner on the phone or a history of ignoring them or trying to avoid payments. 30% of your net income would be a reasonable amount for them to collect.
note that CSA/CMS will want to schedule your arrears over a 2 year period as that is the management steer so you need to do your calculations. if your arrears will not be cleared in 2 years at the level you think you can afford to pay then you need to bypass the case manager and speak to a manager who has more authority to consider the level of repayment of the arrears.
finally, you say you pay a (apparently) high rent so the kids can stay with you. do they stay more than 52 nights per year in which case you will be due a reduction in your assessment. if they don't then you may need to think about downsizing as it seems a false economy to pay for something that is in effect not being used and making savings on the rent will give you extra income to pay bills and treat the kids.