Thanks to the folks at the new tennis app, Tweener, for this simple yet effective consistency drill idea. I tried to embed the actual video but no worky, so try clicking on that link instead.
Players have one minute to hit 40 balls in play. In the video, the player is rallying with the coach. Player gets one point for every ball hit in. The catch is: they can only use one ball. So hitting consistently is key to keeping a rally going and eliminating wasted time chasing down the ball.
For young players you want to avoid anyone standing around waiting in line, even though it is only for one minute. If you have multiple courts available, great - have them pair up and play this challenge in singles half (longways) or full court. If you don't have multiple courts available, consider shortening the time period to 30 seconds, again to avoid lines and waiting. For young beginners I would also eliminate the 40 number and perhaps reduce it or just have the challenge be for who can get the most balls hit in play in the set time period.