The lower paid spouse would have to make over $30k per year just to break even and cover the cost of day care and taxes. To say nothing of the thousands of dollars it costs to work (gas, work clothes/shoes, professional fees, tolls, etc). It seems unlikely that any job paying less than $35k would be worth even getting out of bed for.
When you consider the additional costs of working: no time for making meals and more eating out, no time to do household chores/repairs and possibly hiring someone to do them (or be run ragged trying to do them after work), no time to do money saving things like clipping coupons or managing investments.
Damn, it seems that any job paying less than $45k would hardly be worth it.
And even then, you will be far more stressed and exhausted. To say nothing of the toll warehousing kids in day care takes on them (as opposed to being at home with a mother/father).
It seems that unless the second spouse was a well paid professional, it wouldn't be worth the cost/stress/damage to take a job.
Skipping over the offensive "warehousing" phrasing, as others are tackling that...
Agreed, it is probably harder to clip coupons and make homemade meals when working than staying home, and can be more stressful to juggle things. That said, I think you focus on only on one financial benefit from working, number one of the below list of financial benefits:
1) Wages and benefits (health care insurance, 401k matching, 401k option for tax deferred savings, professional fee reimbursed)
2) Opportunity cost of lost raises cumulating over 7.5+ years (assuming 2 kids, spaced about 2 years about, enter kindergarten around 5.5 years). On the $30k salary you cite, at just 3% raises and
no promotions, that becomes $38k by the end, or an additional $35k in total.
3) Challenges of getting back into the workforce, particularly after a 7.5+ year hiatus (which likely require taking a pay cut)
4) Lost years of credit into Social Security or pension
5) Stability/career for the SAHP if the couple divorces
In addition, working can provide other benefits:
6) Adult interaction for the parents (not all people are suited to stay home full time)
7) Trained child care development specialists (providing care & available for your questions) who are well-rested
8) Regular group interaction/socialization for the child
And don't forget, you still need to buy clothes/shoes, whether they are work related or not. (Likely those requiring more expensive outfits (e.g. suits) are those with a higher salary making it clearly financially better to work.)
Certainly, there are many other benefits to staying home, but a strict look at just the finances should consider the entire financial picture and not just a slice of it.