This is the second reference to baseball in the strip, and the first that shows the kids actually playing. It looks like Patty can really belt the ball, just like her later namesake Peppermint Patty.
It won't be much longer before Snoopy starts using thought balloons, and this joke wouldn't work if they were in use. Did Schulz plan it that way, getting the joke used while the using was good?
These two little girls' opinion of Charlie B. seem to decline over time. I remember, as a kid reading Peanuts compilations, thinking Patty and Violet often seemed incredibly cruel to him. This, of course, is before the diabolical Lucy came on the scene....
Patty gets back at Charlie Brown for past chase strips.
Two things: 1. Charlie Brown's mouth looks a little weird in the last panel. Ever see Grim Fandango? 2. It's a bit less funny when it's a boy chasing a girl after a bad joke.