And does this gal like apple pie and drive a Chevrolet SUV?

First thing I'd been doing is asking your wife what she meant by "
I wasn't surprised." Obviously she's seen or heard things you don't know about. Now's the time to find out.
After all this horny little mother was getting fucked right in public by a guy a lot younger than her. Not many married women with their kids playing ball would fuck under those conditions.
Makes me think it wasn't her first time with this kid. I'm guessing maybe the coach, his wife, and you are late 30's? Might be a good time to plan a little party at your place so the kids can play together and you can get to know them better. Or because of the kids it may be wise to just see what other volunteer work the young guy has done and leave it at that.