> Is there a deeper story that I am missing?

This is how I felt about the episode before I had kids. I thought it was a fine episode but nothing special. Onesies was my favorite. Of course, we wanted kids, so that hit me where I was at the time. Now I have a two-year-old, and I "get" Sleepytime. There is nothing extra to explain except that it will just mean more to you.

Not saying you don't have kids, just my own experience with it.