Thank you for all the thoughtful replies.
My plan was to start a casual conversation about gymnastics if I saw them around somewhere or they asked me a question about our gym switch. I think I would still do this if our paths crossed at any point because, as happychaos and iwannacoach point out, I would really want someone to enlighten me if I were in the same position.
However, a couple years have gone by and there has been no path crossing! So the issue then becomes one of proactivity (do I call/email/send an anonymous note with lettering pulled from magazines

??), which I am not sure I am comfortable with for a variety of reasons. Many of you made some excellent points about why this would be somewhat to very out-of-line.
For the record, I do see these parents as being complacent about taking at face value what this kid's coaches are telling them: it's almost exactly as described in JoyAvenueMom's first paragraph.