The agreement when we split was that we both had both DC half the time and paid for their food / outings etc while with us. Other costs are on a spreadsheet and we settle up whoever owes the other at the end of each month. This covers clothes, shoes phones etc.
3 months ago DD (a teenager) decided she no longer felt able to live with her dad 50% and has since spent 1 night there plus joined him and her brother / his new GF and family on a weeks holiday.
Money is tight but not worryingly so yet but I'm paying for all her food, hot water, hair dryer electricity and trips out with friends - which adds up.
Should I ask if he's willing to contribute? Or point out child maintenance calculator?