Your wife wants to be a housewife, not a stay at home mum, because when there's no child in the house you're not doing the actual job of parenting.
Given that all mums are mum 100% of the time whether they work or not, as a SAHM (rather than a housewife) I look on my role as childcare provider, and if there's no child, and we're struggling financially (or even if we're not but all the burdern is on DH) then I should work.
Unfortunately reality is not as straight forward as that. Because my youngest child is only away from home between 9am and 11.30am. Try getting a job for 2 1/2 hours a day. (The reason I am a SAHM is partly choice, partly a wish to avoid childcare... my choice, there's no right or wrong) Even if I could, well my DH would still have to do the same hours, because there's no way he'd be able to reduce them.
I agree with whitetrash if being a mum is her absolute vocation, if she loves it so much, if you, as a family unit are finding it a stretch to afford nursery fees on top of all the other household bills, then either your wife has to stop the nursery, or get a job.
Please though, even though YANBU, don't dismiss how full on and hard work being a SAHM can be... okay, coffee mornings (you get a coffee break I assume) lunch (same), a timetable that's guided by a small person's needs... all lovely, all perks of the unpaid job... but we don't get sick pay, holiday pay, any pay, the day starts early and finishes late. I've had 'real' jobs too, and in some ways, this is the hardest because we're not stinking filthy rich and cannot afford luxuries (not complaining, our choice).