Once you're looking at a particular game (or a choice among a few), it's pretty trivial to ask the Wal-Mart employee to open the case and let you flip over the box.
Assuming you can even find an electronics associate who isn't busy with another customer. Or knows anything about their department.
In my experience, a parent was either concerned about the content of a particular game ("Joey said to get BloodSlaughter 5 for his birthday - is it appropriate?"), is asking for a recommendation from the staff ("What's good for a 13 year-old boy? Uh-huh, and is there a lot of adult stuff?") or was familiar enough with the games him/herself and not in need of running down each game.
In my experience (and I've been working as a cashier the last ~7 months at a Walmart), parents usually don't give a damn. I cite the mother who sent her ~11 year old to buy his M-rated game by himself (I made him go get his mom before I made the sale... she didn't seem to understand why I needed to talk to her before making the sale). Or maybe they've just made their decision by the time they get to the register. I've really only run into one set of parents that was really knowledgeable about the rating system despite not being gamers themselves.