I think the bottom line is that the CSA amount is a poor standard and that unmarried parents get a very, very bad deal in terms of child support compared to married parents.
I separated from my partner at a time when I was earning £9k per year (I freelance - this amount changed to £20k the year after we split). He was earning £75k. We had two children, then aged 2 and 4.
Nursery fees were £900pcm (this was by far the cheapest nursery in the area we lived in), childcare for after school and nursery was £8-£10 per hour for three hours five days a week, and about £80 per day in the school holidays, when there was neither school nor nursery. My rent was £1200pcm (this is the flat we rented before we split up, and is very cheap for the area we lived in, where rents are more like £2k pcm) - let's say the second bedroom my kids were in accounts for £500 of that. Council tax and heating were £200pcm. Food for the three of us about £200pcm (conservative estimate). I've not included clothing, after school activities etc, and without any of that, the monthly total of the marginal costs of the kids STARTS AT £1300pcm .
The CSA states that on average, couples spend 20% of their income on two children. Now for my ex partner, 20% of his net income less 1/7 (as the children are with him every other weekend) is £750. On £20k per year, £650pcm is about 50% of my net income. With the remaining £650, I am to find £1300 to cover 'my share' of the rent and bills, plus whatever extra is needed to fund extra childcare, clothing and activites.
Is it really right that the state should have been paying me tax credits to cover some of this stuff rather than the father of the children covering a larger protion - something he could easily have afforded to do?
Should I have stopped working until my daughter was at school (i.e. 18 months later) to avoid childcare costs while I worked? But then how would I have paid my rent?
Should I have moved into a different area (possibly a different city, given that London is so expensive), moved my son to a different school, and moved away from my friends and family (i.e. my support network)? How then would the NRP have had the contact he has had with his kids?
Can't help thinking that CSA isn't very helpful, the %s calcualted aren't very helpful and they don't provide a good solution. And I'm surprised that the state is left to provide when an NRP will not.
And to the OP, of course your ex should be paying half the childcare, and of course your new partner shouldn't be responsible for it. And I'm surprised that there's any argument at all.