Government funding covers 30hrs, if you choose a setting that accepts it. So if that nursery doesn’t then fine but use a different one. Most settings cover it now. 80% of the rest of any childcare would be covered by UC. So only 20% of childcare to pay. If OPs children were also 1 that would be different.
I think the point here is there are 4 children. The 2 that currently live with NRP may only have an income from their mum of the equivalent of what RP has. And I mean if evened out, for example if Rp lives in social housing with cheaper rent, gets 25% off council tax, free school meals for eldest, free childcare for 3 yr old, 25% of time without DC there for food etc, and then UC, CB and anything else.
SM’s income may also only cover all the household essentials, leaving all 4 children in the same financial situation. His 2 DC living with him may not be living a lavish lifestyle. Yet he is expected to go to work on top of that to ensure his eldest have more and his youngest have less, so his eldest RP can stay home because it’s easier.
Working and being a single parent isn’t easy but nor is it impossible and on the current benefit system it’s almost impossible to be worse off working because that’s exactly why they changed it, so it paid to work. She has 25% of time without the DC in which to work when NRP is providing the childcare and there is no logistics to consider issue with children’s illness etc etc and all the other excuses. If he currently isn’t having them every weekend, then make it so he is, so OP can work to fill the gap of income.
If you put all 4 children together and 3 parents around them one mum each and a dad between the 4 of them each ‘unit’ should work out the best way to currently meet the needs equally, it may be that in this case the earning has to fall to each mum to do so while NRP provides the childcare to do this.
SM has a partner so not eligible for benefits therefore she must work full time, OP has no partner so could work PT while dad looks after the children (weeekends). Which would be topped up with UC. Both kids (unless SM earns a lot, but even then costs could be higher for house, car needed for work etc etc) should then essentially be in the same position with an income and childcare to facilitate that provided by their dad.