Dave Smith, a successful HS coach and author, talked about non-playing players charting matches. Both kids learn from the experience.
Along the same vein,... recently read "Methany Maniphesto",... pro baseballer's philosophy on coaching (prinicples I've adopted). For kids, in the dugout, he would engage 1 kid per inning, and ask questions about what they would do for various positions,... eg.
* pitcher: what pitch would you throw
* outfield: would you move up or back, why
* infield: play deep, or shallow, if someone on base, where to throw first, etc...
* current batter: any "tells" about their batting?
* etc...
Personally charting matches would be boring, but if I was coaching, I'd ask questions like:
* How is your teammate winning or losing?
* If you played this opponent, how would you approach it.
* if you had to give your teammate advice, what would you give them
* For individual points, ask what good/bad things happened for your teammate
* Any missed opportunities?
* Break points,... what do you think (s)he's thinking right now? Nervous? What would you do?
* Before the point (based on the score), on the serve and return, what should your teammate plan be?
* Count depth of shot, and correlate to who won the point (ok this is in the "charting" category, but more valuable than charting winners and unforced errors)
* Count rally, and correlate to who won the point (eg. establish teammate and opponent's shot tolerance)
* etc...
my $0.02