A simple mistake may leave me in debt

Deductions taken from old employer with minimal contact through myself. I forgot to tell them one of children living with me stopped being a child as he is my “would be” step son. My mistake.

However, my other daughter was being claimed for by me and CSA were notified of the fact so my ex-wife could stop paying for her. They told me yesterday that I had failed to notify them and an allowance had been made for her on my calculation. How can this be?

The underpayment they say is overdue and I have to pay it asap, even if it means taking a loan as they are preferred creditors and must be paid.

Any advice greatly appreciated.

Many thanks,

Simon