The CSA legislation considers that the person who gets the child benefit is the main carer of the children, that is you. If you do make a claim for child maintenance your ex's income is looked at, for 1 child it is 15% for 2 it is 20% of his net income. He would qualify for a shared care allowance for any child who stays overnight with him. The way you describe it he would be allowed 3.5 nights weekly for your older child. The CSA can calculate an assessment and collect payments on your behalf or you can agree to your ex paying the assessed amount to yourself. If he refuses to pay the CSA will enforce payments. If he is employed he will have money deducted from his salary, if he is in arrears this can be as much as 40% of his take home pay. If he is self employed he will be taken to court and a liability order is always granted. This can lead to bailiffs calling on him if he lives in England or Wales, Sheriiff Court officers will enforce payments in Scotland. Legislation allows that they revoke his driving licence and passport, it also can force sale of his home. Non resident parents have also been sent to jail for not paying child maintenance. I have worked at the CSA for 17 years, hope I have helped.