Thank you for your question
My name is Clare
i shall do my best to help you but I need some further information first.
Is it the non resident parent who earns £2,000 a month?
Does the NRP have any other children living in his/her household?
How often so the children stay overnight
So the parent who does NOT live with the children earns £18 a MONTH
he has two children living with him.
How often do his children stay with him?
How many nights/
In that case the CMS (new CSA) guide means that he should be paying in the region of £2167 a month.
You can find the guidelines here
You may also be able to apply for a "top up" if there are school fees to pay
I hope that this is of assistance - please ask if you need further details