If being 'rational' is decided by only the financial calculation, then the most rational decision is to not have kids. Kids cost money, even if in a mustachian way. If it's only about getting enough money to FIRE then one shouldn't have had kids.
If you choose to have kids, then you must also determine what is Best which is subjective.
I agree. Furthermore, if being 'rational' is decided by only the financial calculation, then it's not rational to want to FIRE. Rationally speaking, you should want to save maximally AND work till you drop dead of old age, if accumulation of money is the measurement.
If lifetime total of free time is the metric, ChpBstrd and OP are more rational than their wives.
If happiness of the child is the metric, the SAHM wives may be more rational. ChpBstrd says, "Developmentally, research shows the early socialization and curriculum are beneficial for kids." But, that applies to your older baby or toddler. Young babies don't need much socialization with other children; developmentally they just need to form a strong bond with a loving, constant, and responsive caregiver. A baby can get this from a parent or other relative, an excellent daycare, or an excellent nanny. I personally stayed at home because I know that I'm an excellent caregiver, better than any we could hire, I generally enjoy it, and my husband loves his job anyway. I also calculated that it would take a minimum $60K salary to replace all the home work I do, such as the accounting, cooking, DIY fixing and renovations, etc.
One thing that is bad for children of any age, is to be cared for by stressed or unhappy people, or to be subjected to too much change in personnel. So, a depressed SAHM who never gets a break is a bad idea, so is a pair of stressed-out WOHPs who are running around while the baby is in a daycare that sees a lot of staff turnover, or parked with a series of inexperienced au pairs. Even under these conditions, it's not the end of the world: babies are usually tough creatures. Though some children are more resilient than others (see this article from the Atlantic
for a popular review of the fascinating plasticity hypothesis.)
Anyway, Cheapass and ChpBstrd's desires to retire early thus go head-to-head with their wives' desires to spend time with their infants while they are tiny. The rationality of these desires depends on the metric used. The part which was probably offensive to ChpBstrd's wife is his assumption that his metric is superior.
My impression of the MMM way is frugality
, not cheapness: decide what is important to you in life (obviously an irrational process) and give up things that are unimportant in order to afford the important. Usually this means, give up your credit card addiction in order to retire early and spend time the way you like. But equally, it could mean give up your credit card addiction in order for a parent to stay at home.
TBH, I think that OP's situation (negotiated since they began the conversation) is great... extra time at home, loving family ready to help, plus a part-week work option for his wife.