Costco.
Even if all you bought at Costco was formula and diapers, you'd more than subsidize the $50 membership over the course of a year vs. what you'd pay basically anywhere else.
Of course the economy of scale applies to that membership fee. Scaling across more kids makes a better value prop.
Selection. The brands we are used to is not always offered @ Costco. However, it seems to be much easier to subsidize membership costs with Costco, than with Amazon Prime.
Then stock up, and seriously? Their shelf stocking doesn't work that way. Any SKU ordered (other than food stuffs and specialty seasonals) are stocked for, at least, one month in any given location before long term stocking is decided on
Plus, if you get the Costco Amex card, you'd get cash back. I got $170 back last year with my $110 Executive membership, and I didn't even try very hard.
Of course the economy of scale applies to that membership fee. Scaling across more kids makes a better value prop.