@EarringsandLipstick
She's already said she's happy to have his child 50/50 so she's clearly already agreed to it and knows what it entails.
It doesn't sound like it.
OP's point is that they can't afford the current £250 CM that's paid. Therefore how will they afford all the shared costs - that's clothing, activities, shoes, uniform, school books, presents and so on, in addition to the extra food costs? I don't think this has been considered at all, and it's appalling behaviour by both OP & her husband.
Most of the £250/mo CMS likely goes to cover wrap around child care, or after school clubs. This is assuming worst case financial situation that the ex is single and works FT and has no new partner to split household costs with.
If the father is increasing his care of the child from 1 night EOW to 50% that by itself will account for most of the £250. The ex will avoid those monthly child care costs to her.
I agree the father and OP won’t be £250 better off, they will have to provide more food, some clothes, contribute to uniform, activities etc. Probably extra petrol costs to drive child around (if have a car and live rurally). But I don’t think it will be a full £250/mo out of their pockets.
I think overall the ex’s cost avoidance plus getting 50% support on daily costs will be higher value than £250/mo and thus him being a SAHD will be of benefit to his ex, not a detriment. It is also, in best interests of the child to be raised by their father 50% instead of 1 night a week. They are bound to feel left behind and replaced by new child if the father keeps it at 1 night per week.
I agree if budget doesnt balance, it’s the fathers responsibility to make them balance and not the OP. Through part time work, using his savings...whatever. I just think that the SAHD idea could be a win win for everyone involved, especially the child who would benefit from increased contact with their father.