In this early baseball strip Charlie Brown's team is losing even though he's catching this game.
Look at that first panel for a moment. There are six kids pictured. Counting from the left, it's hard to tell who the second and third ones are. The second kind of looks like Charlie Brown, and the third like Shermy, but they're already in the shot viewed up close. This would make these two kids the first "extra" human characters in all of Peanuts. (Animals have had a couple of extras so far, an anonymous dog and a bird.) Some of the other panels have unknown extras too, as well as Lucy and Schroeder a couple of times.