They’re crippling the already crippled

I am resident parent of 2 kids. I earn minimum wage and get wtc and ctc for 1 child, the second child is 19, just finished college.

I have another child who is staying with her mother most of the time, but not all the time. She has no residency order.

The CSA say I have to pay £25 per week for the child that lives with the mother. And I cannot afford it with 2 children to pay for out of wage (which pays for our home) and wtc/ctc which pays bills /food, with very little over for the fuel and car to take daughter to school / other daughter to this and that.

Ex wife has managed to lie that the child living with her has lived there since Jan 2010, but really she moved there in July 2012. Thus they seem to believe I owe them £1350. I have just been threatened with enforcement action, to which I have responded by telling them the information again(Stonewalled thus far) and asking them for a full breakdown so I can highlight the wrongdoing.

The CSA employee dealing with it seems to be completely clueless.

What I cannot understand is I am making a home for my 2, and third one when it suits her. And these charges will cripple an already crippled family unit, ruining the children psychologically in the process.