Sears: think Walmart but without a grocery section LONG before Walmart. Used to be catalogue only. Big box warehouse basically
K-Mart is cheap, shitty Target, basically. Always felt that way to me, anyway.
So the joke is that most normal babies come in a kit from the Sears catalogue (or box store) but Calvin was a cheap, Store-Brand deal that's almost as good for half the price!
Explain like I'm Calvin is a community like Explain Like I'm Five. Only instead of providing a simple easy to understand explanation, it's explained in a silly nonsensical (and incorrect) way. Much like how Calvin's dad often explains things to Calvin.