I was looking at this site http://consumers.quickstarttennis.com/ and the only problem I see with it is that if the kids hit the ball past the other kid then they have to go chase the ball. Doesn't sound too fun to me.

If there was a back wall then that would alleviate that issue. Sure they wouldn't be able to play past the baseline, but as a fun game plus getting experience hitting over a net - I think it'd get the job done.

I wonder if this would even be a fun game for adults to play...