Maintanance And my boys?

I am currently have my 2 boys.. Mon to Friday from 8-5 and then mostly sat and Sunday's aswell.. I have them 1/2 nights a week.. I work nights so im already losing money there but there my kids so it's worth it..

My ex works through the day and had a comfy 30k plus job.. But she's demanding £100/200pm off me for the kids? Saying I dont contribute towards them?

Am I in my right to refuse to pay?
Maintanance And my boys?
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